Silver dollars fighting 10/7/2009
Hello,
I have four silver dollars
<Groups of six or more, please.>
in a 72 gal. They have been together a little over two months. (Other fish: 7 Serpae tetras (starters), 1 turquoise rainbow-3.5", 1 male pearl gourami-4"-starting to become more feisty, 1 platy-2"(starter), 1 bushy nose pleco-4", 1 clown pleco-2.5", 3 bamboo shrimp).
<An interesting mix of fish. Serpae Tetras tend to be nippy, so watch them carefully. The others should be fine, though it goes without saying that as schooling fish, Rainbows are happier in mixed sex groups of six or more.>
The tank has been running almost 5 months and the water quality is good. I do a 10-15% water change weekly, run two 350 Marineland filters, keep the temp. at 79. Two of the silver dollars have red anal fins (I guess they are called anal fins, they are the fins underneath the fish towards the back),
<Yep.>
and the other 2 don't really have any color on their anal fins.
<Hmm... if these are Metynnis argenteus or some other Metynnis spp., then it's the males that have red anal fins.>
One of the red anal fins is 4" in diameter and the other is about 3.5". The other 2 have the same measurements. About a month and a half ago, the two colorless ones developed 2 black spots on their sides and the edges of their tail fins/top fins also turned black.
<Do look at Metynnis maculatus, which has black spots on its flanks.
Metynnis argenteus sometimes has very small speckles on its flanks, but not really big spots. Metynnis lippincottianus is another option too; while similar to Metynnis argenteus, it tends to have bigger spots on its flanks. Myleus rubripinnis is also sold as the Silver Dollar sometimes, though its red, hook-shaped anal fin should help tell it apart. Spend a little time on Google comparing photos. While I'd be surprised if you had multiple species in one batch of fish, it's not impossible.>
They have been acting more aggressive towards the other 2 silver dollar, with the bigger one being the most aggressive. These two have often been rubbing against one another. I'm confused, I thought the ones with the red anal fins were the males, but they get chased into corners and they stay out of it when the other 2 are rubbing and fighting.
<Well, yes, the ones with red fins usually are males if we're talking about Metynnis argenteus.>
Are the 2 with the black spots both males fighting for dominance, or male and female?
<Difficult to say. When kept in groups less than six, "all bets are off" when it comes to behaviour: schooling fish simply don't work as they should in too-small groups. In a nutshell, schooling fish try to establish a pecking order. In big groups, no one fish can bully all the others all the time, so a sort of peace reigns. But if you have too few, you make it easy for a bully to cause trouble, and their schooling instinct doesn't work.
Try adding some more and see what happens.>
They are often going at it while the other 2 pick a corner until it's calm again. I have also noticed that the 2 with black spots will periodically go over to the other silver dollars and nudge or nip at it, then go off again to rub and fight. This has been going on for the past 2 months. I assume it is some kind of breeding behavior, but shouldn't it be over with by now? I have not seen any eggs, will they just keep going until some eggs are deposited.? I wasn't to concerned before, however, now I'm starting to see more ragged fins with no end in site. Should I let the behavior run its course, or do I need to do something?
<Do something.>
Thanks for any info
<Cheers, Neale.>
Re: silver dollars fighting 10/7/2009
Thanks for the quick response,
The tetras have been behaving and only fight amongst themselves and the Gourami has taken over the tank.
<Well, okay.>
The Gourami doesn't beat up any one fish but they all know whose boss.
<For now.>
I attached a few photos, maybe they can help with identification.
<Actually, they look rather odd, and not any species I'm familiar with.
Myleus schomburgkii has a black vertical band on the flanks, but a bit further back along the body. They certainly aren't the Red-Hook Metynnis, Myloplus rubripinnis; the anal fin is all wrong for them. The two common Silver Dollars, Metynnis hypsauchen and Metynnis argenteus, have the right body shape and the red anal fin, but not the spots on the shoulder like yours. Metynnis maculatus has lots of spots, all over its body.>
The two that develop the spots (also red throats and black edges on fins) don't always them. They seem to change almost instantly when they are ready to do battle, which most of the time during the day. I was afraid of adding more because I didn't know if there was enough room left. I'm not getting a bigger tank any time soon, however, if adding more with bring peace, then I'll do it.
<I suspect adding a couple more will be helpful.>
Thanks again
<Cheers, Neale.>