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I have 1 female RES in a 20 long tank that I keep just under half full (anything more than that and she becomes "Adventure Turtle"). There is one of those crappy turtle docks, a heater and a Duetto 100 filter that is mounted on the bottom of the tank and she places it where she feels it would work the best although I try to keep it at the back of the tank.
I am getting a Fluval 205 filter from a friend for free and am wondering what the best arrangement of media would work the best. This is my first canister filter so I want to set it up right.

I have a fluval 404, which is a larger (older) version of the 205. There's 4 baskets in mine, but if memory serves, the 205 has 3.
In the bottom basket, I have prefilter, next is carbon, and the top two baskets are biomedia.

As for the Adventure Turtle, you could get a screen for the top of the tank (make sure the holes are at least 1/2" so the uvb can penetrate). That way you can give her more swimming room