When I had water turtles, and I brought the smaller ones in for the winter, I had them in a 60 gal preformed pond in my basement, and I brought the filter in too and used that. I don't remember the name of it though.
Look for a fountain filter like those for whiskey barrels (Pondmaster 190, 190gph) @$50. That's what I have used successfully. Not the easiest to clean every week but, it did a great job cleaning without the horrific currents. Even guppy feeders could swim by. Replace the fountain pipe with whatever you want. Just don't give the turtles a chance to get caught in the initial exhaust flow. You need a good muck filter to deal with turtle waste so as to prevent it from creating an unnecessary bio-load in the process of bioremediation. Lots of surface area for the nitrosomonas bacteria to colonize.
Below is the 90 gallon I set up for the baby Snap (serpentina serpentina) I named Cora. Greek for little jewel as most little girls are. Did you see her? Anyway, the pics show the basic layout before the pond plants arrived and the fountain pipe adapted to flow through the refugium. The pond plants helped keep the water clear. Both floating plants and potted ones were used. A double strip light with Desert 10.0 UVB's did the trick. The gravel is medium river rock, $5/20pounds from L o w z.