The C4 would probably work for some freshwater fish. The big problem is that most pet shops don't have a clue how much waste a turtle produces in relation to fish. A single turtle poop from a medium sized turtle is more waste than a very well stocked fish tank produces in a week.
I'm glad the filter is working out for you. (I knew it would)
And your initial observations are correct... it breaks in. When you first turn it on, the fx5 sputters, and very slowly begins to increase flow. Its actually working out the trapped air pockets in all of the various media. Make sure your valves are fully open. The FX5 once broken in (sounds like yours is) puts out quite a bit of water. It's 600+ GPH at head. pretty amazing filter for around $200.
Post some pictures if you can.