06-02-2010, 06:30 PM
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The surface area is significant as is the length. For instance, shoaling fish like tetras, rasbora, danios, etc need swimming room (some more than others) so a 20 long works better. The surface area is also greater with a long than a high, and this affects the gas exchange (oxygen in, CO2 out). Some fish have certain needs with respect to water flow and oxygen exchange. As you have the option of either, I would suggest deciding first on the type of fish then buy the appropriate tank for them.
Which brings us to: The water parameters matter because not all fish can live in the same water, as I expect you know. It is easier and less problematic to acquire fish that are compatible with you water rather than having to adjust the water to suit certain fish. This is sometimes easy, sometimes not and can be quite expensive long-term. But again, depending upon what you have out of the tap (which is usually the cheapest source considering regular water changes) and what fish you might like, there may be several options.
Once you/we know the water parameters, then you can narrow down possible fish; and the type of fish you decide on will determine if the long or high is best.