Are water changes really necessary?

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If a tropical tank tests okay, the water is clear and the fish and plants are healthy, is there really any point in doing routine water changes?

Technical expert Neale Monks says: Under lab conditions, scientists have indeed maintained balanced aquaria (called microcosms, or else mesocosms if the size of a small pond or bigger) where nothing is added or taken away on a routine basis. The idea being that by examining how a given ecosystem works in a lab, it’s possible to understand how ecosystems in the real world work, where observing the actions of things like algae and mud-dwelling invertebrates would be much harder to do. These systems have been run for years at a time, perhaps periodically topping up with distilled water to compensate for evaporation, but that’s about it.

In theory at least, if a home aquarium was run this way, you wouldn’t need to do water changes. You can imagine a situation where nitrate and phosphate were kept below safe levels by balancing production of these (by the filter bacteria) with the uptake by plants and algae. Crop the plants back and compost them, and you’d effectively be exporting nitrate and phosphate. Similarly, evaporation of water over time would be precisely compensated for by adding distilled water as necessary, which would keep water chemistry steady.

In practice, though, this doesn’t seem to work in home aquaria. It’s hard to say exactly why, and that’s perhaps because we test only a few environmental parameters. Even if we export nitrate and phosphate, what about other ions, such as sulphate? Few aquarists measure the total dissolved solids in their tanks, and even those that do aren’t able to pin down the exact cocktail of different minerals. General hardness, for example, includes a variety of calcium and magnesium compounds.

On a week-to-week basis, any small variations caused by animals and plants using these minerals doesn’t really matter because the next water change will restore the levels. But if the tank is left alone indefinitely, without monitoring all the different dissolved minerals, or solutes, you could end up with a deficiency in one or more of these, posing a danger to your livestock.

Another question is what happens to the buffering capacity of the water over time. This is the intrinsic ability of a given sample of water to resist changes in pH, usually because it contains dissolved ions that ‘mop up’ the ions that cause pH to rise or fall. Even across a single 24-hour period, the pH can rise during the day (when CO2 is absorbed by plants doing photosynthesis) and fall during the night (when they’re not). This effect can be dramatic, and as a rule, we don’t test the pH every few hours through the day and night, so have no idea about how much it changes during such a period. While we know that fish can handle slow or slight pH changes with varying degrees of equanimity, rapid or dramatic changes in pH can be stressful, even lethal if large enough.

Finally, there’s the accumulation of what are loosely called tannins. These are dissolved organic compounds that build up in water over time. Some are directly excreted by your livestock, but most come about from the decay of organic material. Tannins are what turns water brown when it flows through peaty soils, resulting in the famous blackwater swamps and streams so popular with advanced aquarists. These compounds are frequently acidic, and so have the potential to lower the pH. Again, water changes, plus the buffering capacity of the water, tend to slow this down so we can usually ignore it. But if the tank is left indefinitely, these tannins build up, and the pH starts to drop. Irrespective of the preferred pH levels of your fish, a pH below 7 will slow down biological filtration significantly, and below 6, largely stops altogether.