I've said it before and I'll say it again. Turtles don't turn the food they eat into more poop than a fish would. Does that make sense? Someone said earlier that they had 10 adult snappers in a 100 gallon and had to change the water 75% every day. Doesn't surprise me at all. That was probably about 60-80lbs of turtle in there. Imagine if you had ten - 20"+ arowanas in a 100 gallon (a 20" aro isn't even close to the mass/bio load of a adult snapper, but just for sake of comparison), how bad do you think the water would be? Not very drinkable... right?
I keep my turtles just as if they were fish and never have any issues with water quality. For example- I have a 50 gallon tub (about 40-45 gallons of water) that houses 3 RES (5-7") and the filter I have is a Cascade 700 canister (185GPH- for tanks up to 65 gallons). I change the water every 2 weeks(30-50%) and open the filter every 2 months. They are on a bare bottom and waste doesn't really build up. If it does I just scoop it out. The turtles are happy and I have not smelt "dirty turtle tank" since I set it up.
I think one of the main reasons turtle tanks get dirty is overfeeding (same as a fish tank). I feed my turtles for about 10-15 minutes every day. After that, any uneaten food is removed. Its simple, don't leave food in the tank, and keep a W/C schedule.
I do have to say I don't like Hang on back filters for turtles for many reasons. One- they don't have very much biological filtration. Two- turtles tend to push up the the down tube and that leads to burnt out pumps. Three- HOBs limit your water level. You need to have the water up to a certain point for it to draw water up without to much strain on the pump. Canisters work with the help of gravity and use the pump to return water to the tank.
I use canisters and wet/dry filters for my turtles. I am not too fond of HOBs (the reasons I stated earlier) and sponge filters for turtles (can be bit/chewed off and swallowed- I don't know if this could hurt them, but I won't risk it). Anyhow, just keep your turtles water as if it was a fish tank and you/your turtle will be fine.
One more thing... you don't need to feed your turtles every day. If they are on a healthy diet a little break from food will be good every now and then. And it will cut down on your waste. I do like to feed real young turtles every day, but after the 4" mark I start to cut it back. I feed my adults every other day.