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Yes because it means you took out the impurities. For fish that like soft water it's fine as is, but for fish that like harder water you may need to add something like salt for example. It depends on the individual fish species though.
That's what I was worried about, I don't know what minerals etc might need to be added back to RODI water for a freshwater community. Could it be too pure and be missing something that is needed? i know most just use tap water with an ammonia-chlorine remover like Prime but I would prefer to use the RODI that I already have unless it is harmful in some way to be that pure.