500GPH peak is correct, rated up to 110G (supposedly). I run these on 2 55s, but believe in redundancy (and overkill) so I also have XP3s on them.AC 110 = Aqua clear 110 (old 500), I assume. I think they are rated up to 100 gallons.
I think it is also 500 gph but I may be wrong...[FONT=arial, helvetica]
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My preference would be two, but that's because I believe in backup, in case one filter fails when the pet store is closed. (Which, by a law of the universe, is always when that happens. :mad2i am ordering an AC 110 for my 55g malawi tank which when completed will have about 10-15 fish. is 1 enough or should i order 2?