Hmm.... considering a fluval 4 is made for tanks up to 100gallons you'd think it would do better than that.
In my opinion, if the turtle tank gets to be more than 40 gallons then I start using pond filters just simply because they're more reliable and a lot better with the whole biological/mechanical filtration process. I've always used pond filters with my turtles and they've always kept the water crystal clear indoors... outdoors I also use a UV sterilizer to keep the water from turning green with algae. Anyway, even with a pond filter there will still be a bit of sediment that will build up on the bottom if it doesn't get sucked up in the pump but if the circulation is going good then hopefully the waterflow will carry the majority of it and the pump will suck it up and send it to the filter. Are you using gravel on the bottom of your stock tank? That will help make the water even more funky as it collects muck from the turtle.
Aquarium-type filters just don't cut it for me very well when keeping turtles. The fluvals I've found are the better ones, but still. Otherwise I can't recommend any other aquarium filters that will do better than the fluval.
The only thing is not only do you have to buy the pond filter but you also have to buy a pump of the appropriate gallons per hour (gph) for the filter and tubing to fit it. The pumps aren't that expensive though (to me anyway). Also with filters that have UV sterilizers, the UV helps to kill harmful bacteria as well.
Here's the pumps I recommend and have used for years, I would suggest 140gph or 250gph:
http://www.petsmart.com/global/produ...N=2030157&Ne=2
Here are some filters, and remember they have to be set up at the height of the top rim of your stock tank so the water can flow back into the tank:
http://www.123ponds.com/an-226.html (with UV)
http://www.123ponds.com/an-228.html (without UV)
http://www.123ponds.com/tp16783.html (Tetra brand that I'm currently using with my turtle ponds indoors right now, works awesome)
Here is some non-kink spiral tubing; although I think each one of these is more than you need you may be able to find tubing at Lowe's, Home Depot or a pond store near you that has tubing. For the filter outlet (where the water exits back into the pond) it's best to have some tubing on those along with hose clamps, and you'll also need a hose clamp for the inlet as well. When trying to find the right diameter tubing for the inlet and outlets on the filters, measure the
inside diameter of the tubing and the outside diameter of the inlet and outlets on the filters. I hope that made sense.. lol
http://www.123ponds.com/plumbing-sup...al-tubing.html
123ponds is a good place, a little expensive maybe but they have more filters and different types of tubing for you to look at...
I know this is a lot of info but in my opinion the pond filters are worth it. Let me know if you have any more questions.