Combining Filters
Aquarium Setup #82 – October 1999
This article first appeared in the October 1999 issue of FAMA. Copyright © 2025 by Steven J. Schiff. All rights reserved.
Last month we began a discussion of the technique of combining different types of aquarium filters in use on a single tank in order to maximize filtration effectiveness. I gave the example of using two AquaClear outside power filters (OPFs) on my 20 gallon tank, and filling them with an unconventional complement of media. This setup gives me greater versatility than if I had but a single filter on the tank, and it compares favorably with many canister filters that are recommended for this size tank in terms of media volume, water flow rate, and cost.
Filter combinations are not limited to OPFs. All of the many different types of filters can be combined with other filters on the same tank. If you combine two filters of the same type, you get “more of the same” – i.e., a greater volume of water filtered in the same way as a single unit would. However, if you combine two dissimilar types of filters – for example, an OPF and an undergravel filter (UGF), you can use each type to offset the weaknesses of the other. In this example, the high flow rate of the OPF teams up with the superior biological filtration performance of the UGF, providing an all-around excellent filtration setup. Remember, it’s not important where in the system various filtration functions are performed. Therefore, letting one filter concentrate on one type of filtration, and another filter concentrate on another type, is a perfectly valid way of setting up your filters for maximum efficiency.
Some types of filters are generalists, excelling at all three (mechanical, biological, and chemical) types of filtration. Examples of these are OPFs and air-driven box filters. Other types of filters are more specialized, and excel at one particular filtration task at the expense of the others. For example, diatom filters provide unequaled mechanical filtration performance, but their biological filtration ability is negligible at best. A diatom filter run full-time on an aquarium, especially as the sole filter, would be a poor choice. However, combine the diatom unit, used part-time, with a full-time UGF, and you will have crystal-clear water that is also free of harmful ammonia and nitrites.
Most filters are not at one extreme or the other, being neither highly limited specialists nor completely undifferentiated generalists. Most perform at least some mechanical and some biological filtration, and many of them can provide chemical filtration if set up to do so. In fact, the way you set up the system contributes substantially to its characteristics and capabilities. A little planning beforehand will let you reap significant benefits in terms of filtration efficiency and effectiveness.
The simplest type of filter combination involves installing two or more of the same type of filter in a single tank. This allows you to provide the required amount of filtration in discrete subunits, rather than a single large block, so you can tailor filtration capacity more exactly to your individual setup. (Admittedly, this is not a very big problem to begin with, since there is a lot of flexibility in the guidelines for “required” filtration for a given aquarium size. If you “overfilter” your tank, you’re not likely to be hurting anything.)
Another advantage is that multiple filters provide you with some protection against failure of any one of the units. If one does fail, the other (or others) can still provide some filtration for the system until you can fix the problem. This is similar to my recommended practice of installing two heaters in a tank, each with half the recommended wattage for the tank, instead of a single full-power unit. If one of the lower-power units sticks in the ON position, you will have more time to discover and correct the problem than if it happened to a full-power heater. Conversely, if one of the half-power heaters fails totally, the second one will still heat the tank at least partially, again providing you with time to prevent disaster.
A further important benefit of multiple similar filters is that you can clean them in sequence, rather than all at the same time. By doing this, you avoid disrupting the all-important biological filtration of your system. If you have a single OPF, say, and it uses a filter cartridge for mechanical and biological filtration, then its biofiltration capabilities come from nitrifying bacteria which colonize the solid surface of the fibers in the cartridge. When you change the cartridge, you discard all of the beneficial bacteria that have taken up residence there. If this filter is the only source of biofiltration in your tank, then you are discarding the only thing standing between your fish and ammonia poisoning! Don’t do it! If you have more than one such filter, you can clean one at a time, allowing continuity of filtration while the cleaned one is “disabled.”
It’s very common to run more than one OPF on a single aquarium. This allows you to add filtration capacity as your finances allow, for example, or as the stocking level of your tank increases. One very effective setup that I’ve seen in many breeding tanks is to have two, three, or more air-driven box filters running simultaneously. These filters are cheap, reliable, and effective, and they are safe for breeding tanks – they don’t suck the fry into intake tubes, or produce strong currents that can sweep babies away. You can use as many, or as few, as required for the size of the tank and the number of fish involved.
Combining different types of filters on a single tank is a bit more sophisticated and it requires knowledge of the different characteristics, the strengths and weaknesses, of the different filter types. Generally, you will combine a filter that is strong in one area and deficient in another, with a second filter that has complementary capabilities. One example is the diatom/undergravel combination mentioned earlier. Another would be an OPF set up for biological and mechanical filtration, combined with a canister filter loaded with peat moss or dolomite, to alter the water chemistry and provide chemical filtration. (I usually don’t recommend that you perform in-tank chemical filtration, but not everybody listens to me anyway!) Since, as stated above, most types of filters are “generalists,” there will usually be some overlap in the filtration provided by dissimilar filters. This is something to be welcomed, rather than avoided, since it provides a measure of redundancy to your setup that is helpful if there is a partial failure. Take a cue from the Space Shuttle – it always pays to have backup systems.
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