Filtration and water partials are two entirely separate things. That seems difficult to grasp for many hobbyists. So this brief note is aimed at clarifying the roles of filtration and water changes.
Filtration has three specific jobs it can do, each with its own name:
1. Biofiltration is to reduce the toxicity of nitrogen-containing wastes (ammonia initially, then nitrite), oxidizing them to less toxic nitrate.
2. Mechanical filtration is for a) water clarity, b) for current for exercise, c) for oxygen absorption from the air above the tank plus its distribution throughout the water volume, and d) similar CO2 off-gassing.
3. Sometimes chemical filtration is used for special or specific material removal purposes, such as medications or dyes.
Water changes other than when cycling with fish are not for really any of those things above. Water changes during cycling with fish are to protect the livestock and are required because biofiltration is not yet well established in that tank. In other words, in such cases the water changes temporarily substitute for biofiltration.
Water changes are to:
1. Reduce the buildup in the water of organics and inorganics that are part of normal tank processes far beyond source water levels of those materials
2. Restore used-up minerals that have been depleted by normal tank biological processes.
3. Remove excess minerals built up from foods and wastes.
4. Restore, at least in part, the water’s original profile.
These are not interchangeable lists or functions – you really cannot trade off one against the other, or use excess of one to compensate for lack of the other. The only trade-off possible is (the almost never practiced by hobbyists) use of flow-through systems, which reduce or remove the tank requirement for filtration.
It is not rare for comments to show up on the forums about not doing many water changes because the tank or tanks are heavily over-filtered. But that statement or the reverse one of doing good water changes so that they do not need filtration, just do not make sense in either option.
Biofiltration, in the end result, increases tank pollution by the production of nitrates. But nitrate is better than ammonia, as it is far less toxic. So we are happy for that trade-off.
Mechanical filtration removes particles from the water column and that increases water clarity. That is a positive result. But many to most of those particles are organic, and will be biologically broken down by bacteria and infusoria unless you rinse filters very frequently. That also increases tank water pollution. Daily rinsing is good, but how many of us do that? So, mechanical filtration can also contribute to water pollution by dissolved minerals and organics from the breakdown of organic particles. But it does capture particles and make them easily removable when replacing or rinsing the mechanical media. It is still a trade-off, but has an overall positive effect on the water quality and appearance.
Chemical filtration sometimes is beneficial in removing certain dissolved materials such as certain medications, or dyes used as medication, or other dissolved colorants such tannins, which can be present in fairly high concentrations. Water changes may be as effective, or more effective in such removal. You need to judge the effort involved against the cost of the chemical filtration technique used.
Water changes are a broad-brush technique. They remove pollutants and restore reduced or depleted water profile items in exact proportion to the percentage change done on the tank. If you do a 50% partial water change, you reduce the detectable (nitrate for example) pollutants as well as the undetectable ones (dissolved organic compounds) by the same percentage – 50% in this example. You also restore depleted minerals (such as carbonates/bicarbonates) by the same percentage.
The combined result of filtration and water changes is hopefully to maintain our water as close as we can to what we started with originally. But neither of those things can do the job alone. It takes both filtration and water changes to get the job done.
Think about it.
This note originally appeared on another site. It has been edited for use on this site. Sept. 2010.
Robert T. Ricketts, a.k.a, RTR