02-16-2012, 07:17 PM
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Normally I would not suggest a larger filter than the size recommended by the manufacturer for the tank size. Sometimes this may be necessary, but not here. The flow will obviously increase with a larger filter, and you don't want to be buffeting the fish around. Rainbows may prefer a gentle current, but not a raging river. Fish have to continually spend a lot of energy just to "fight" currents that are too strong. I certainly agree on 50% weekly water changes.
On your pH issue. The difference between 7.5 and 7.9 will not even be noticed by the fish. Ranges for pH have to be given as a guide, but it is understood that they are just that, guides. Provided extremes are avoided. And pH 7.0 is a significant point. Below 7.0 is acidic, above is basic, and this does make a difference to fish. So your rainbows that have a preferred range of 7 to 7.5 will be fine if your tap water is 7.9. What happens in the aquarium depends upon the hardness, both GH and KH, of the tap water. The pH may lower as the tank establishes, or it may remain fairly stable.