How to extend the water changing time in an aquarium for two juvenile red eared slider turtles?

I rear two juvenile red eared slider turtles in an aquarium about 3’ x 2’ x 1’ in size. The problem is, as they get bigger, the aquarium water gets dirty pretty quickly. When they are small baby turtles, I only need to change the water once a month. Then the water need to be changed once in two weeks. Now, the water starts to get cloudy in a week time. Any solution to extend the water changing time?