This is a discussion on overfiltering? within the Beginner Freshwater Aquarium forums, part of the Freshwater Fish and Aquariums category; -->
Is it possible to over-filer the water? Like getting a canister filter that pumps 160 gph on a small 20 gallon tank?...
Well, at some point you're going to be creating so much water movement it's uncomfortable for the fish, but other than this, no, you can't really overfilter a tank. For example, I have a 29g tank that houses a single 4.5 inch Jack Dempsey (he'll get upgraded to something bigger, don't worry) and is running a Fluval 204 canister (180 gph) as well as a Skilter 250 Power Filter (250 gph) for a total of 430 gph on a 29g tank, so I'm turning that tank over about 15 times per hour, which is three times the "standard" amount of 5 times/hour that is what most filters are rated for.
If your fish look like they are in a whirlpool, then you have a bit too much power! Basically you want to find a balance specific to the fish you have where you have happy fish and a clean tank. For example, my betta's tank has a tiny little azoo (turned all the way down) which is only rated up to 5 gallon tanks and the tank is 5.5 gallons. I am very happy with the results and my betta is as happy as can be with the low flow rate. On my 15 gallon planted tank though I have a canister and a HOB filter both rated for tanks up to 20 gallons. So, I've got over twice the filtration and it works fine because this tank is a lot more "messy" mainly because I'm a novice plant grower. But, none of my fish in there have long, flowing fins that would cause them to have difficulty swimming.
In short, it's all about knowing your fish and finding a balance.