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/The Conscientious Aquarist

 5 Pros & 5 Cons of Undergravel Filtration

Bob Fenner

Undergravel filters, uninstalled

            I was blessed to have James Lawrence of Microcosm Publishing as my editor/mentor while penning 'The Conscientious Marine Aquarist' in the mid-nineties, though we did have some decidedly contrary opinions re gear selection'¦ particularly the use of undergravel filters!

            'Those things (U/G) are old, outdated, a nuisance to maintain'¦' James position'¦ I told him, 'Look', if a customer comes into a retail store with only so much money, looking for a 'complete' set-up, freshwater or marine, the retailer is not going to tell them: Gee, you don't have enough money for an outside power filter, or protein skimmer'¦ or such, but sell them what WILL work for the given sum they have on hand'¦ Very often this will be an undergravel filter!' The DO work'¦ though those rascally plates and various types of riser tubes, beneath tank drillings, other means, air and power to move water through them'¦ do have their shortcomings and downsides. 

            Well, here are mine (and David Boruchowitz's, TFH Editor) reasons for and against (yes, you can'¦ and if you're old/experienced enough, should) these 'ancient' yet still functional modes of providing physical and biological filtration in captive aquatic systems.  

The Pro Arguments for Undergravel Filters (UGFs):  

1. No Moving Parts, Simple To Install & Operate: 

            What could be better? No moving parts (usually), generally no other gear to buy other than the occasional pump diaphragm, maybe some airstones'¦ Compared with other filter types (power, canister, sump, box, sponge'¦ outside/inside) UGFs  are a breeze to install and keep up. No expensive, messy filter media'¦ your gravel/substrate is about it'¦ and this can be easily vacuumed (or for folks with 'reverse flow' arrangements a twist of a well-placed drain valve'¦) while doing your routine frequent partial water changes. Ahhh! 

2. UGFs are Invisible:          

            Like children and the I.R.S., 'out of sight, out of mind' is a byword of UGF usage'¦ like a stealth pooper scooper, these devices suck and hold onto detritus, uneaten food, excreta'¦ and aid in their conversion to liquids, gasses'¦ by chemical and biological means. Part of the 'greatest story ever told' in the way of nitrogen et al. cycling is the boost in added circulation through the substrate that undergravel filters provide'¦ The forward reaction of nitrification, the oxidation of ammonia to nitrite then nitrate via helpful aerobic (using oxygen) bacteria made far more efficient by having water drawn through or pushed between substrate particles, home to bazillions of such microbes.  

            Alternatively and in addition you can elect to use bulky, unsightly filters of other sorts'¦ sitting in your tanks, hanging on the back, as squatty canisters, or placed as sumps and such below and above your systems'¦ how ugly!

3. UGFs are Effective                 

             The single greatest cause of captive aquatic life is without a doubt  'metabolite poisoning''¦ simply put, the toxic accumulation of animal wastes'¦ ammonia and nitrite principally. This was the case in times of Romans throwing their misbehaved slaves to their Morays, the corner-store goldfish and its attendant bowl, and will likely apply to our antecedents on distant planets in the future'¦ Undergravel filters are superior in their capacity to convert such noxious wastes to less deadly formats, and other than fluidized bed filter types, are best able to 'ramp up', rise to the proverbial occasion of increased metabolite presence'¦ from overfeeding, livestock death'¦ 

Compared with most all other filter modes and media, UGFs are fastest to become populated with desirable microbial life, resist crashing (through filter media changes, medication mis-treatments,  too much livestock added too quickly'¦) and recover from challenges to your needed bacterial complements. 4. UGFs are Inexpensive to Acquire & Operate: 

It almost sounds like a TV commercial, but there is 'nothing else to buy', they are 'freeway close', there are 'thousands sold in Europe''¦  Okay, well, those last two statements are like a TV commercial. Undergravel filters are so simple that most people could build their own if they were so inclined. Contrast this with making any other filter type! Slotted or otherwise perforated grate, perhaps covered with chemically inert screening'¦ somehow raised up to allow a little water space underneath, and some mechanism to draw or push water through them and the gravel above'¦ that's about all there is involved in undergravel filters'¦ unless you fashion or are sold fancier models that incorporate chemical filtrant holders on their risers.  

Compared with filter media purchases for other types of filters, their greater electrical costs to operate'¦ and much shorter effective useful life-spans, UGFs are hands-down winners. 5. UGFs Are Versatile---Use With Powerheads, etc.         

            If you don't have much money to invest in buying fluid-moving pumps, air-lift systems can be easily and efficiently employed to drive your undergravel filter. Need or want more water flow? Powerheads of various capacities can be utilized to either draw water through (normal, regular or gravity flow) or push (reverse-flow) the water through your UG plates and substrate.  

            Don't have room/place for chemical filtrant use with undergravels? Actually, Zeolite, carbons et al. can be easily and tidily enclosed in porous filter bags, a bit of the substrate swept to the side and the filter media laid out of sight but still in use over the filter plate.  

            An added level of functionality can be enjoyed by placing a layer of 'filter floss', aka Dacron polyester media over your filter plates, sandwiched twixt the plates and gravel substrate. This added layer really boosts your systems capacity for nitrification, whether utilized in a fresh, brackish or marine setting. See Ostrow (1981) re.  

And Undergravel Filter Downsides:  

1.UGFs are Hard to Maintain, Especially Long-Term 

            'They're like sweeping your dog/cat poo under the rug', bad visual, but accurate to some extent. Unless you're careful about regularly vacuuming your substrate, moving the decor about, and the occasional complete or nearly complete tear down, 'gunk' (a polite term) does tend to accumulate in the gravel and particularly under the plates of UG filters. 

            In fairness, all types of filtration have their respective maintenance and up-keep issues'¦ and if folks are careful enough to not overfeed, overcrowd, ignore their regular maintenance, undergravel filters can be run safely for years and years.  

2. UGFs Can't Be Used With Diggers 

            Eartheating Juraparoids and many other Neotropical cichlids, 'engineering' gobies of many sorts in marine systems, and many other animals that burrow extensively can create 'channeling' problems with undergravel filters, digging out the substrate, allowing water to easily pass though their tunnels, while dropping circulation through the rest of the filter 'bed' (the substrate) to dangerous anaerobic levels'¦ with these choices in livestock, you're well-advised to at least use other types of filtration IN ADDITION to your undergravel plates.  


3. UGFs Can Produce Noxious Gases--Creating Dead Zones 

            Most everyone who's been in the hobby long enough can relate an incident of 'rotten egg' smell (Hydrogen Sulfide) wipe-out in an aquarium serviced with undergravel filtration. These instances are almost all matters of inadequate maintenance, where the aquarist neglected regular maintenance, fed too much of inappropriate foods, had too much bio-load'¦ reminds me of folks getting defibrillated for myocardial infarctions (heart attacks), 'If you would have stuck with your 12 point maintenance program we wouldn't have to jump start you.' Honestly, if you're marginally diligent in gravel vacuuming/water changing, stirring your gravel'¦ and other simple aspects of proper aquarium husbandry, stinky UGF wipe-out/dead zones are highly unlikely.  

4. With UGF Use Fishes Can Get Trapped Under The Plates  

            'Where'd my Clown, Kuhli Loaches, Gobies, Blennies'¦. go?' 'Let's see, there not carpet jerky'¦ I bet it's that darned cat!' There are a host of fish species (and some invertebrates) that delight in hiding out under UG filter plates'¦ the good news is that most all of them are just as facile at coming back out'¦ If you're missing some livestock and suspect that it's ensconced under your UGF don't panic, don't tear the tank apart'¦ very likely your aquatic charges will re-emerge on their own'¦ when they're hungry. 

            At least with undergravel use you have less chance of your livestock 'jumping out' through extra and larger openings made to fit hoses and attachments, necessary with other types of filter use.  

5.Undergravel Filters May Present A Problem With Plants 

            Many rooted plants (as opposed to floating or surface types) have difficulties with too much water circulation as produced with undergravel filter use'¦ growing slowly or not at all due to the aerobic changes that such gear induces. Is this a big problem? Not at all! Swordplants, Crypts, Vallisnerias, Sagittarias et al. can be easily 'blind potted' in trays, clay pots and such or an area left open (sans an undergravel plate) for their planting'¦ or a section/plate of impervious (likely plastic) sheet inserted over the undergravel plate where you want to put them. So there! 


            Okay, so I'm an old timer who still likes, can see applications for undergravel filters in modern aquarium keeping'¦ Though they do have their attendant downsides, given regular upkeep, a practical understanding of their potential troubles, mis-applications UGFs can be at least useful adjuncts to other types of filtration. They do work! 

Further Reading: 

Channel, Robert, 1987. Floggin' that old horse again. FAMA 10/87.  

Dewey, Don. 1979. How-to: Here's how to build your own highly efficient undergravel filter for those large tanks, or the odd sized aquariums for which no standard size filter is available. FAMA 6/79.  

Dow, Steven. 1991. Educate your staff on undergravel filters. The Pet Dealer 12/91.  

Edmonds, Les. 1989. Understanding the undergravel filter. TFH 1/89.  

Goemans, Bob. Undergravel filters. TFH 4/01. 

Helwig, Dan. 1983. How-to: Construct undergravel filters. FAMA 2/83. 

Hovlid, Norman G. 1959. The case for the under-gravel filter. TFH 7/59.  

King, John M & William E. Kelley. 1973. Efficiency of subgravel filters. Marine Aquarist 4:3, 73.  

Nichol, Red & Jim Cooper. 1978. The undergravel filter: the debate goes on. FAMA 4/78. 

Ostrow, Marshall E. 1981. A filtration system with a difference. TFH 9/81.  

Parker, Nancy J. 1974. Eggcrate Filter. Marine Aquarist 5:2, 74.  

Schiff, Steven J. 1990. Undergravel filters: maintenance and alternatives. Tips and techniques for obtaining efficient, effective biological filtration for aquariums. AFM 5/90.  

Schiff, Steven J. 1993. Undergravel filters, pt.s 1 & 2. FAMA 10 & 11/93.

Volkart, Bill 1991. Understanding undergravel filters. TFH 6/91.  

Watson, Thomas T. 1983. Understanding undergravel filtration. TFH 6/83.

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