04-04-2010, 02:48 AM
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Well RO water is basically incredibly soft and usually has around a neutral pH. If you have hard, alkaline water, you can keep fish that come from those types of environments but might have difficulty keeping fish from soft, acidic water (especially sensitive ones). With really soft, neutral water, your pH is naturally going to fall a bit (although this can be helped along with something like peat used in your filter) to achieve very soft, acidic water that would be perfect for keeping certain fish.
If you've already got the RO machine your possibilities are basically endless. I'd have a look at our list of fish profiles here
and see what sorts of fish strike your fancy.