Welcome.

Your turtle is a RES? Your turtle's shell length without the curve is nearly 6 inches? If so, ideally he should be in a 60 gallon tank.
With the size you say your turtle is and with the size tank (20 gallon) he's in, I really don't think any filter is going to work that well in terms of keeping the water clean- without frequent cleaning--he's big enough to be producing a lot of poop.
For the type of filter you're describing, an Aquaclear is good, but the largest one (the 110) I think would be too powerful for a 20-gallon tank.
What would really be good is to start with a bigger tank/container and get a good canister filter.
I don't think the Turtle Reserve is the best thing to be using to create a basking area for your turtle (so it's good you took it back). It would take up far too much room in an already-too-small tank. Your turtle doesn't need a reserve in the tank. A basking area that allows for swimming room underneath to maximize the small space he has would be far better.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-