If you could provide us with a picture or more details on what exactly you're working with myself or others can probably give you more specific answers.
Sumps generally have a few sections/steps.
1. Physical/Mechanical
2. Biological
3. Return Pump/heaters/equipment
For physical you can do a number of things ranging from floss pads to micron socks etc, it really depends on what you want to do. I will make one recommendation though, make sure it's easy to clean or you wont do it and your pets will suffer.
Biological can be done a few different ways as well, scrubbies(cheap), bioballs(slightly more expensive), ceramic rings(more expensive), K1, and there are probably other ways that I haven't heard of. Biological filtration can be done in a few different ways, fludized beds, wet/dry, and wet filtration. I don't know much about fluidized beds, but from what I understand about wet/dry biological filtration and wet biological filtration, the wet/dry is more efficient than wet filtration, so if you can go that route I'd suggest it.
Some people also chose to run carbon, and if you do choose to go this route make sure that you do it right and run it actively. Run it in a media reactor such as the Two Little Fishies Phosban Reactor 150 with a Maxijet and you're all set for pretty cheap. There are more expensive options out there, but in my opinion they're not worth the extra money unless you've got a larger tank in which case I'd just suggest going with Two Little Fishies Larger Phosban Reactor the 550.
I'm not really sure how this would apply to turtles, but I do know that turtles are insanely messy compared to fish, so I'd suggest beefing up that physical/mechanical filtration and cleaning your sump more often than recommended to ensure that it doesn't turn into a nitrate factory as the waste decomposes.
Check out this site
http://www.melevsreef.com/allmysumps.html
He's got a ton of information on sumps and refugiums, but in your case you probably want to do more of a sump since I think you're doing fresh water and don't necessarily need pods(food) for your fish.