04-20-2010, 11:50 AM
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Potentially large fish (and 11 inches is large) require adequate tank space throughout their lives. It is not onlythe physical space (in terms of the area) but the water quality that exists in that space. Both of these, and especially the latter, influence the growth (and health) of a fish throughout its life. As a fish grows (which is does all its life), its internal development continues. The quality of the water affects this development quite a bit. It is for example possible to raise fish in smaller tanks, but it requires extensive water changes to maintain the water quality. Filtration does not achieve this. Jack Wattley has written of raising 75-80 1-inch discus in 20 and 30 gallon tanks--with partial water changes of 50% several times each and every day.
A filter only circulates the tank water through media. The larger the filter the more media so the more space for nitrifying bacteria. But this is only a small part of the issue. The water is still the same water, and there are all sorts of substances in the water from each of the fish. Some of these affect different fish in different ways. Which is why it is so important to carefully select tankmates.