12-17-2009, 05:21 PM
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Angel's last sentence is quite true, as explained below. I found my last post on this, so here it is copied over, with a few additional points.
The first thing to remember is that in nature fish are continually in "fresh" water. The water is being turned over (as in ponds and lakes) or constantly flowing past (as in streams and rivers) and the percentage of fish mass to water is considerably less than in any aquarium. No fish is forced to live in the same water, so right from the start our aquarium is at a disadvantage for the fish.
The only reason to do a pwc in a planted aquarium is to rid the tank of toxins that build up and cannot be effectively removed any other way. These toxins are urine and solid waste from the fish, and they are significant; a small tetra can produce its body weight in urine within 3-4 days. No filter will remove this, period. Plants can, but it is a slow process and only effective if the fish load is very minimal and there are many plants. One author used the example of 6 or 7 neon tetras in a 55g tank that was heavily planted as being the upper limit. Most of us have far more fish in our tanks that this, so we need to do the pwc to remove the pollution. If a well-planted aquarium has a small fish load, fewer pwc's will be needed; Diana Walstad writes of doing one every few months, and that works if the fish load is not beyond the capacity of the plants and biological system. Again, most of us have more fish than the system can support without our assistance via the weekly pwc. In non-planted tanks, the pwc also dilutes/removes nitrates, but this is irrelevant in a healthy planted tank because the plants consume the ammonium, and nitrates are therefore minimal.
It is frequently said that the pwc should be more frequent with less water in order to sustain stability in the water quality. In a planted aquarium the plants are doing the major filtration and the water is, as I've indicated above, going to be stable if everything is working the way it should. So that leaves us with the pollution (toxins). The more water changed, the more pollution is removed, plain and simple.
In the November issue (2009) of TFH there is a good article on this. The author ran tests and explains why changing more water is preferable to changing less water. Pollution accumulates daily (the waste from the fish is steady) and every day an equal amount of waste is added. In other words, the toxins are increasing far more as each day goes by, so each day there is a high percentage of pollution in the aquarium. Changing 50% once a week is cutting the pollution in half, with the result that day by day the pollution will gradually increase toward the end of the week; in other words, the fish are only going to be subjected to very high levels of pollution at the end of the week just before the 50% water change, so during the previous days they are exposed to slightly less pollution that they are with a twice-weekly 25% water change. Of course, changing 50% or more each day would be ideal. But most hobbyists can find it easier to maintain a regular weekly schedule rather than a daily one.
Coming back to the water stability issue: there is no logic in maintaining more stable pollution in a tank. No one could logically dispute that reducing pollution is a benefit and the more the better. At the same time, a significant weekly water change will actually work to maintain more stability long term in the water parameters.
To sum up, a weekly pwc is the minimum in an aquarium, and changing 50% will be healthier for the fish.