just clean change the water more often...everybody wants an easy way ourt of water changes for turtles...soem buy huge tanks/ponds
some use chemicals
soem spend zillions on filters
i have doen all of these
(i like many users here have LOTS of turtles) i have found that the only way to really keep the water clean is frequnet partial water chnages.
My girlfriend and I once decided to pay a high school student to come and clean some tanks because the amount we were spending on filters and media and the time we were spending doing water chemical tests made paying somebody actually cheaper..
just change a little of the water alot.