Best filter for turtle tank?

For smaller tanks/turtles, internal power filters or submersible filters would be a good idea. These filters are able to be utilized in shallower water than traditional power filters (the ones that hang off the back of tanks.) I would also recommend that you get a filter that is rated for a larger tank size than what the turtle is in; turtles create A LOT of waste so the more filtration the better. You can't over filter a turtle tank. Here's a couple examples of internal power and submersible filters:

http://www.amazon.com/Tetra-25817-Whisper-BioScrub...

http://www.amazon.com/Fluval-A470-U2-Underwater-Fi...

As the turtle grows and if you end up with a 40 gallon tank or larger, I would recommend that you switch form the above mentioned filters to a canister filter system. These filters are made to handle larger volumes of water and waste; those smaller filters just don't cut it anymore with larger turtles. Again, go big or go home with your filter choice. Bigger's always better. Here's a couple examples:

http://www.amazon.com/API-Filstar-XP-M-Canister-Fi...

http://www.amazon.com/Marineland-C-220-Multi-Stage...

Here's a few reliable places to read about aquatic turtle care:

http://herp-info.webs.com/aquaticturtles.htm

http://www.austinsturtlepage.com/Care/caresheets.h...