03-09-2011, 03:35 PM
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Filtration should be based according to what is in the tank, meaning fish and plants. You don't mention either, so it is hard to say yea or nay on this or that filter.
I have a Rena XP3 on my 115g and I never saw anything so good, except the Eheim of course. But then, it is very heavily planted, with about 140-150 fish, smallish, and you don't need mega filtration with such a setup. It removes so much stuff from the water I have to rinse it every 6-8 weeks; my Eheims on similar tanks go 3-4 months, but the Rena will very noticeably slow down if left past 2 months due to the stuff it is catching. My water is crystal clear.
If your 125 is planted, with forest fish, the Rena will be perfectly adequate. On the other hand, if it is plant-less, with larger fish that produce more stuff, you may need more. I won't offer advice in this case since I have never gone down that road.