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FAQs on the Red Eye (Moenkhausia) Tetras

Related Articles: Cardinal Tetras; A School of Beauty, Part II,  by Alesia Benedict, Characid Fishes

Related FAQs:  Cardinal Tetras, Characid/Tetra Fishes,


Diamond Tetra swimming erratically      4/14/19
Dear WWM,
Hope you are doing fine. Thank you for maintaining this excellent site, a great help to us hobbyists.
<Thanks for the kind words.>
Question: One of my 6 Diamond Tetras have started swimming erratically since yesterday. They are established fish, in the tank for almost 2 years.
There is no bloating, no injury marks, no loss of sparkle, no excess mucus and all the fins are intact. It just kind of topples over or at times fall sideways, try to regain posture and the cycle repeats.
<Understood. This does happen, and usually indicates environmental distress if sudden. There's no real treatment, beyond ensuring stable, ideally optimal conditions. Filtering the water with carbon should help, and if the carbon is older than a couple weeks, then change it. Carbon should remove things like insecticide sprays that can be used in the home but unfortunately are highly toxic to fish. Otherwise, this sort of erratic swimming can indicate Whirling Disease (Myxobolus cerebralis) but this is extremely rare in aquaria because of its complex life cycle. Of course erratic swimming can be a symptom of things like White Spot, Velvet, Dropsy, and Finrot, but it should be obvious if this is the situation here.>
Tank is heavily planted 29 gallon, in its 8th year. No Ammonia or Nitrite, Nitrate 25. Filtered by three 500l/hr HOB filters filled with sponge, bio balls and ceramic media. Also a small bag each of Seachem Purigen and Carbon. Another pump works a chiller. pH 7.6, TDS 300ish. Tank mates include other tetras, Pencilfishes, Corydoras, BNP, Ram cichlids, all other fishes looking absolutely fine.
Points worth mentioning: I had a sudden surprise attack of Ich after years, 2 weeks after I introduced some Cardinal Tetras which themselves had no visible spots and were claimed to be quarantined by a reputed seller I have dealt with in the past. The spots affected only some Green Neons. I went the high temp way and allowed the tank to reach 31 C over the last 5 days.
The spots are almost gone now. No fish is showing any signs of Oxygen hunger, no gasping at the surface, there is good circulation and surface agitation. I usually maintain the tank at 25.5-27.5 degrees C with a chiller (this is an Indian summer).
I fed freeze dried Tubifex worms yesterday, which I do rarely. Else their diet is from various flakes and pellets from Tetra,, Ocean Nutrition and Hikari.
I added the regular weekly dose of Seachem Flourish comprehensive and Flourish Iron yesterday, a practice I have followed for years. I also add 5 ml of a DIY macro mix every other day as per the recipe from James's Planted Tank.
No use of paint fumes, aerosols, pesticides, etc. near tank. I have made a 40% water change today but no improvement in conditions.
I replenished my dried Almond leaves 5 days ago, these are self collected.
Is this whirling disease?
<As stated above, extremely unlikely. The parasite needs other animals such birds in the environment for the life cycle to complete, which really isn't likely indoors!>
Or some kind other neurological issue?
<Certainly a possibility, if some toxin was present in the environment.>
Maybe the heat stress?
<Can be, though usually when tropical fish are exposed to cold. When too warm, you normally see gasping behaviour rather than loss of motor control.>
Do I euthanize in case it is contagious?
<Almost certainly not contagious.>
Thanks in advance and keep well.
Devakalpa, India
<Hope this helps. Cheers, Neale.>
Re: Diamond Tetra swimming erratically      4/14/19

Dear Neale,
Thank you for the detailed reply.
<Most welcome.>
Unfortunately the affected fish passed away last night.
<No surprise, really. But always hopeful!>
I isolated it after mailing to you. Rest are fine as of now.
<And to you, too. Cheers, Neale.>

Re: Update on fish tank/ stocking questions. Random... Moenkhausia     5/3/14
Thanks so much for your information Neale. Could I up the school of red eye tetras or would that be pushing it. Right now its 11 fish, I had a dozen one died. I was thinking of getting 5 more.
<I would not... experience has told me that oftentimes large groups of fish die back to the number the tank holds easily. So leave things be for now. In any event, for a ~50 US gallon tank, a group of a dozen Red-Eye Tetras sounds about right.>
I was wondering are African butterfly fish compatible with red eye tetras or would they get ripped, how about Indian butterfly catfish.
<Red-Eyes look nippy (menacing jaws) but usually aren't if kept in adequate numbers. But you can't be 100% sure with any fish.>
Also I saw a T5 ultra strong spectrum bulb light for growing live plants. Are you familiar with this? its quite expensive the t5 lighting but according to the staff at Tropiaquatics it will allow me to grow more types
of aquatic plants
<I don't know the specific tube, but generally a good quality T5 tube with the right colour spectrum (6500 K for example) can be expected to grow plants well. Probably need 2-3 tubes the length of your tank for medium-bright lighting. Cheers, Neale.>

are red eye tetras a good fit for a 53 gallon, 36 L by 20H by 15W    3/7/14
Tropaquatics has a 10 for $20 special on them. could i get 20, with 2 rams, a L204 Pleco 2 cories.
<If you mean the South American Red Eye Tetra, Moenkhausia sanctaefilomenae, yes, this is a great species that does well across a broad environmental range (it's one of a few South American tetras that genuinely thrive in hard water). The only downside is the species can be nippy, especially when bored. Fine with Corydoras, Loaches, L-numbers, Danios, etc but less good with Dwarf Cichlids, Angels, Gouramis and the like. Cheers, Neale.>
Re: Blue eyed Pleco/ cloumbia info needed.   3/7/14

So they wouldn't mix well with Bolivian rams then?. What about keyhole
<I would only mix South American Red-Eye Tetras (Moenkhausia sanctaefilomenae) with other boisterous and active species. They aren't nasty fish, and not dedicated fin-nippers in the same way as, say, Serpae Tetras, but they do get bored easily, especially if kept in too-small a group. In a mixed community of loaches, catfish, Danios, barbs and of course other tetras, they're a hardy and adaptable addition. If you do try them with medium-sized cichlids like Acara, do keep a decent number of Red-Eye Tetras, do give them space, and do keep an eye open for any signs of fin damage. Cheers, Neale.>
am I overstocked
Mas childish blather       3/9/14

I have a 50+ gallon freshwater system. 15W 36L and 20 H. In it I have 12 Red eye tetras, hope to increase the red eye tetra school to 20+, an L204 a pair of Bolivain rams. and 2 surviving sterbai corys. which i hope to give back to the LFS.
the tetras aren't that nippy and seem to fit in really well just got them yesterday though.
<Glad they aren't causing any problems. Keep an eye out. Tank doesn't sound overstocked. With small fish like these, the "inch per gallon" rule works reliably well. Cheers, Neale.>

Tetra issue. Moenkhausia injured, living solo  11/14/11
Well I'm not entirely sure what to do about this fish. Early September, I was placed in charge of a 55 gallon tank here at my school, first thing I did was get the water levels how they should be.
<Good start>
There were small amounts of ammonia and nitrite (less than .5 ppm, but I was not happy with that)
<Me neither>
and the KH was non existent while the GH was way over 30 (I stopped counting is was so bad). This tank is stocked with tetras, patties, barbs,
<Mmm, what species are these Barbs?>
mollies and Pleco. Everything seemed just fine, I started putting AquaSafe in the tank (slowly to ensure the fish acclimated correctly, they had been adding straight tap water for over a year and the Ammonia and Nitrite levels here are insane).
<In the source water? This is bad... would not use for my potable purposes w/o running through a reverse osmosis device first>
Everything was fine, we lost one fish (had been purchased from Wal-Mart and it looked slightly discolored when I started). Well, about a week ago now, I noticed that there was this huge blackish/brown spot on the right side of the Lamp Eye Tetra. It was swollen, so I pulled the fish and it has been in quarantine ever since. I watched it the first couple days and smaller spots began to appear on other areas of the fish, so I started adding Rid-Ich
<Mmm, I wouldn't do this. The active ingredients are too harsh... Formalin and Malachite Green>
(which has worked on everything I've ever come across without issues) and the smaller spots went away, but the swelling on that one spot has continued. I've never had anything require treatment for more than four days, so I stopped treating yesterday.
This spot now has some slight red around the edges of it now.
The fish is eating and swimming fine, but I'm not sure what else to do.
<Nothing... other than providing good care... water quality and nutrition wise... Likely this Tetra has been bruised... perhaps from an altercation w/ another fish here, maybe just a "bump" into something hard. Will heal on its own in time>
Nothing has changed in the tank in a month and a half and I even added a female Molly and a Dwarf Gourami to the tank (quarantined and added after the tetra had been removed, so they did not cause this). This fish is about 2 inches nose to tail and they are expecting me to magically cure this fish, but I'm unsure what to do for it right now. At this point, I don't think it's Black Spot. The only time I seen something even close to this before was on a female guppy who developed an internal tumor, was black and swollen, she died within a month.
Current tank stats:
Ammonia: 0, Nitrite: 0, Nitrate: 5, KH: 2, GH: 29, pH: 7.0, Temp: 72-74 degrees (depends on if the lights are on).
<Mmm, given the mix of species, this water is fine>
Thank you!
Confused Student.
<And a note re this fish, Moenkhausia spp. are strongly schooling species.
Really will do much better kept in a small group. I'd add a few more. Bob Fenner>

Please help! Tetra hlth. 9/10/09
I looked on your page to find out some info on my little fish. He is a red eye tetra and for the past 3 days he has had something resembling the shimmies and cannot swim. He lays on his side or back at the bottom of the tank breathing very slowly and when he tries to swim or move he looks like he is having a seizure. He is getting very small because he can't eat and I'm not sure what I can do for him. He doesn't have any discoloration and the other two fish in the tank are normal. We have a few other neon tetras and I changed out 20% of the water yesterday and the temperature is correct for the fish tank. Please let me know if there's anything I can do.
<Hello Caroline. With very small fish, symptoms such as these are not promising. The first thing is to check the aquarium conditions. Do a nitrite test and a pH test, just to make sure everything is okay. Yes, it's
only one fish sick *now*, but if he's the smallest or weakest, he might simply be the first of many. Next, see if you isolate the fish. A quarantine tank is ideal, but otherwise, a floating breeding trap is useful. Put the fish in there, and observe closely. Sometimes, fish go "loopy" if exposed to a short-term shock; I've done this to my fish one time adding freezing cold water (obviously, without thinking what I was doing!). As they warmed up, they settled right down again. So, if there's something amiss, perhaps water quality, a sudden pH change, or the use of a pesticide spray in your house, water changes will help, and you may see the
fish improve. All this said, with very small fish there's usually little to be done. Do have a read of the best euthanasia methods, so that you're prepared to relieve suffering if it doesn't look like he'll get better.
By the way, Red-eye Tetras (Moenkhausia sanctaefilomenae) are somewhat hierarchical and can be nippy; keep in groups of 6 or more specimens to prevent bullying. Some tetras will turn on one another if kept in too small a group. Cheers, Neale.>

I need help/advice; Poecilia, compatibility   7/29/08 hey! umm.......well, my oldest guppy's tail is being nipped at. His tail looks pretty bad. Could this be a case of tail rot and I do not know about it? or is a species of fish in my tank picking on him? in my tank I have: Guppies Zebra Danios Rasboras neon tetras red eyed tetras glow light tetras mini catfish I have two gigantic zebras and two smaller ones. I put the big ones in my breeding trap to separate them from Flame (my old guppy). but, they have been ing there for about a week and his tail is still getting nipped at piece by piece. Before I had the big zebras in the breeder, Flame's tail had gotten nipped all the way back to his body. It has grown back some. but I can still see that his tail is getting torn up. oh, I have had Flame ever since October. -Sarah <Short answer is that Flame Tetras (Hyphessobrycon flammeus), Glowlights (Hemigrammus erythrozonus), Rasboras and Danios are rarely fin-nippers. But Red-Eye Tetras (Moenkhausia sanctaefilomenae) are known fin-nippers. I have no idea what "mini catfish" are so can't comment on them! You will need to treat with a Finrot/Fungus medication promptly to PREVENT a secondary infection: untreated, your fish can become infected and sicken. Something like Maracyn (popular in the US) or eSHa 2000 (my preferred option here in the UK) would work well. Avoid salt or Melafix -- these have little useful function against Finrot. Cheers, Neale.>

Red Eye Tetra/Molly mix    1/19/06 Hi there, <Morning> My husband recently set up a 10g aquarium to which we introduced 3 red eye tetras. We thought at first that we had two females and a male, but now it seems more likely to be one large female and two smaller males. They seemed happy enough, so two weeks later (three days ago) we brought home three black Lyretail mollies, two females and one male. The introduction seemed to go off without a hitch... However, now the one female tetra chases the two smaller male tetras quite a lot, and I think the male molly might have Ick! To top things off, my daughter and I just noticed 6-10 molly fry darting here and there. So my dilemma is that I don't know what problem to see to first! Should I go and pick up another female tetra (or two) to balance out their need to school, <Two would be my choice... an odd number... ones of about the same size... and may not work. But hold off till there is no ich problem> and will treating the Ick when the spot falls off of the male molly harm the molly fry? <Possibly, yes. Mmm, you should know that the two species you have do not share much overlap in water quality preferences... the tetras like warmer, softer, acidic water, the mollies, hard, alkaline cooler water... with some salt content... which the tetras don't appreciate... Would be much better to have these in two separate/different systems...> I don't want to introduce too many fish, nor too soon, but will the minor aggression I've noticed just continue to escalate? <Yes, likely> I also believe that the mollies will be happier with a few more females, but since there are fry, I'm hoping that at least one or two will survive without any special care. (So far they seem to be ignored by all the adult fish) I would appreciate any advice you can give me on what course to take and when. Thanks so much for your help....great site!! Cathy <Thank you my new aquarist/friend. Do seek out a "less toxic" treatment for the ich... My fave: Aquarisol and elevated temperature... to the low eighties F. Good luck, life. Bob Fenner>
Re: Red Eye Tetra/Molly mix, ich  1/20/06
Thanks for getting back to me! <Welcome> Unfortunately, I had the chance to make things worse before I heard from you... I went and picked up another black molly to try and calm the male down, and three more red eye tetras to try and calm the female down! <Not with the ich in the system? And odd numbers, a surfeit of females are better...> It's made a small difference in temperament, but I've most likely gone and overcrowded my 10g tank now. (6 tetras-all different sizes and 4 mollies, 1 male, 3 females...and don't forget 11 molly fry, 2 days old! Ahhhhhh!) Now I can see that I've created a bit of a mess, but I can't correct it until I treat for ich, right? <Correct> Thanks for the recommendation (Aquarisol), I will pick some up ASAP. Should I treat immediately, or wait for the spots to fall off of the male molly? <Treat immediately> I've only detected two so far. I read that the things in the spots must be "free swimming" to be effected. (more bad advice?) <Is so, but one cannot see these other stages... and you need a therapeutic dose present at all times to eliminate them then... otherwise multiple generations become established... covered on WWM> Luckily my husband has agreed to setting up another 10g aquarium (bless him) so that I can separate the mollies and the tetras and add some salt for the mollies. The one I brought home last night looks a more vivid black than the ones I've had a few days, and it was bought from the same tank. I really like both species and don't want to get rid of either. <Good> I thought I was doing so well researching all of this on the net, but I guess I found out the hard way that not all advice is created equal! (Including the stuff they tell you at the pet store!) <Amen... have found some quite contrary and insufficient information re other topics, fields...> Thanks so much for your time, it is much appreciated! A fumbling newbie, Cathy <You have a good, discerning mind. Take your time here and you'll do fine. Bob Fenner>

Baby red eye tetra... comp.   2/7/06 Hello, <Hi there> I have searched the internet to no avail and would really appreciate an answer to this question.  I have a 10 gallon tank with 2 red eye tetras and another tetra of similar size (though I do not know the type.) The tank has marble like glass on the bottom, and is well planted with artificial plants and coral (lots of hiding spots.)  Several months ago, I noticed a tiny little fish in my aquarium, with the characteristic red above it's eyes.  I bought a small breeder contraption and put it in the tank, to allow the little guy his own space and keep him safe.  He has grown to about an 3/4 inch long, but is still thin and tiny compared to the adults in the tank.  My question;  when is it safe to return him to the main tank with the adults?  I am so amazed by him and don not want him hurt by the adults! Thanks for your help, Faith <Good question... a lot depends on the size, make up of the system... as many small tetras/Characoids can be quite nippy. I suspect that this fish at three quarters of an inch is ready... Would just keep your eye on it/them for possible too-much nippiness. Bob Fenner>

Murderous Red-Eye?  1/1/06 Four days ago we set up a 20 gallon fresh water tank with a filter, heater, bubble stone, fluorescent light, and some Cabomba.  After a day, we introduced 4 Zebra Danios, a Black Skirt and a Berry Tetra, 2 Red Eye Tetras, 2 male Dwarf Gouramis and a glass catfish. <It seems wrong to add 8 fish to a tank that has not cycled yet, actually it is wrong> Initially, the Berry Tetra swam near the tail of the Black Skirt and seemed to want to school.  After a day the Black Skirt went from cool to aggressive towards the Berry, darting and nipping at it whenever it approached. Also, the two larger Zebra Danios chased the smaller two and the larger Red Eye chased the smaller one and occasionally chased the zebras. <IMO black skirts can be real tough guys> The smaller Red Eye began to hide in a corner behind the Cabomba, with the glass catfish. I added three pieces of Texas Holey Rock to break up the space and give the fish more places to hide.  This seems to have increased the territorial behavior.  The smallest zebra began hiding in the corner with the smaller Red Eye, who occasionally would chase it out.  Since then, the larger Red Eye has chased the smaller one out of the corner and taken it as its own, and mysteriously last night the smallest of the zebras lost its tail fin and ended up belly up in the tank.  At this point it seems like I should segregate the larger Red Eye for a day or two in a bucket.  It seems too early to add more fish -- the tank hasn't cycled yet and ammonia, nitrite, and nitrate are still undetectable.  In a month or so, would adding more fish help with the mix?  I have read that tetras are less aggressive towards each other in larger groups, but would I be setting the stage for some kind of gang warfare if I added more?  < No more, let the tank cycle> Also, crowding can by itself bring out aggressiveness in fish, and I don't know if I'm already pushing the limits on crowding.  Another issue -- the tank's pH is now 8.0, almost certainly because of the limestone.  It was 7.4 prior.  I love how the aquarium looks and really don't want to take out the rocks, but I don't know how my fish will do long-term is such an alkaline environment.  Any advise would be appreciated. < Please read here http://www.wetwebmedia.com/FWSubWebIndex/fwset-up.htm keep a careful eye on your fish, read as much as you can, there is much to learn > -- Jennifer McDermott <regards, Joanne>

"OSCAR" 08-2000 - 02-2002... rest in peace O.K. I'm crying about a fish.. My little red eye tetra (his name is Oscar) I've had since I started my tank died and I am just overwhelmingly sad.. I need to know if it was my fault.. I noticed he wasn't looking so hot the other day.. He wouldn't eat and he was not swimming well and God bless her, the head light tail light tetra (Ginger) was trying to help him by nudging him around the tank.. I started treating him with Melafix fix  <a nice move... I like this tonic just fine> and by the time I got home tonight he was floating and barely breathing so I am euthanizing him now in the freezer. Before a couple of days ago he was one of my fastest fish.. Him and Ginger and my Tiger Barbs would chase each other all over the tank. I had had some fish disappearing and found out through my pet store that the catfish I had were the likely culprit, so they have been traded in.. I also got some great advice from y'all and have been trying to adhere to it.. Water conditions have not changed. Ammonia, non existent, ph 7.5, temp 78 20-40% water change every 2-4 weeks. I fed a diet of shrimp pellets, micro pellets, Tubifex and blood worms alternating and being careful not to overfeed.. I did get a nitrate tester on the advice of my pet store and it said my nitrates were high, but I called their advice line and they said that that is normal in a tank w/ biological filter and actually healthy and not to worry about it.. They said they are finding it is common in home aquariums.. He did look a little beat up. (bottom side of body somewhat red, so were gills.. Looked like some scales were missing) Could he have gotten in a fight w/ the catfish or the barbs?  <possible, but old age is more likely. Most tetras only live a few years and you bought him as a young adult, no doubt... indeed two years is very good for some tetras> He did seem to chase the barbs a lot, but they seemed to get along.. Of course when he started swimming slow, one of the barbs nipped his fin. not bad though, just a little nick. He was a year and a half old and I read his life span should be five years. I'm really upset because he is one of my original fish.. Now I'm worried that the head light tail light tetra won't fair well, because they were always together. I have a 20 gal tank now w/a silver tip shark, three barbs, 2 common Plecos and a rhino Pleco a green Severum cichlid and the tetra and up until today Oscar, the red eye tetra *sniff* Do you think I have room for one more tetra to be the survivor's tank mate or does she need one? and why did Oscar die? *sniff* <honestly.. I don't think any tetras belong in your tank... the Severum and rhino Pleco grow to be monsters! You need to pick a theme for the tank and stick with it, my dear> Sorry for the long email.. Trying to avoid going to the freezer and disposing of his remains. Anyway, even though I'm bummed I love my aquarium. I love to just sit and watch it and want to make sure I take care of it.. I am sending two pictures if you want to see them.. <a lovely and fun little fish. May I suggest that if you are sensitive to such losses that you stick with cichlids like your Severums... they can easily live more than ten years! Kindly, Anthony>
Oscar 8-2000 to 02-2002 II
Thank you so much.. Feeling much less emotional today. Perhaps not a good idea to send an email right after you've scooped the fish out of the tank *grin* I am going to take your advice.. I originally got my two tetras to see if I could care for a tank properly as they are pretty hard to kill.:)  <indeed...lovely and hardy fishes, albeit naturally short lived> and have been moving up to more difficult fish.. I really like the fish I have now and think I may end up getting more barbs, couple of cichlids, my shark and my Plecos when I get a bigger tank. Does that sound good?  <absolutely... as long a space allows> Of course I'll read and research each species before adding them  <you do yourself and your charges a great service and respect in so> (learned my lesson on that the hard way!) Anyway thank all you guys for answering all my questions.. YOU ROCK!!! <a pleasure. Kindly, Anthony>

Sick tetra? I have a red eye tetra that's ballooned up on the under side; is this a pregnancy or a bladder or swim bladder disease? <Hello...Jorie here.  I really can't say what's going on without some more information. First off, how big is your tank, how many other fish are in it (and what type), and how long has it been setup and running for? Have you tested the water recently for ammonia, nitrite and nitrate? If so, what are the readings? (Ideally, all should be at zero.) Also, what is your fish's behavior like to make you think it could be a swim bladder disorder?> It had no swim difficulties... <This likely rules out swim bladder disorder> and gills quickly <Do you mean rapid breathing? I'm not quite sure what you mean.  If it is rapid breathing, do test your water for ammonia, nitrite and nitrates, as mentioned above, and do a water change to get those readings to zero if necessary.> What should I treat it with? <For now, I'd say nothing. Do a water change and please try to give me some more specific information about the tank, other fish, and the affected fish's behavior so that I can better help you.> Many thanks, <You are welcome. Good luck.>

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