I think that that mini canister filter would work- as long as the input was low enough in the water that it didn't run dry and you had a way to connect the output to the back of your water feature it would be fine. I've had my eye on these for a while, personally, but haven't yet taken the plunge- it would sure get the water clean, and you wouldn't have to bother with hiding something in the tank. There's also this one that has smaller in and out lines, and so might be even easier to use:
501 Turtle Canister Filter
The second one doesn't look like it has an output you can connect anything to- I think it's just for moving water around.