Lets ask the normal questions first.
How big is the aquarium, and how big are the turtles.
I am assuming since the water area is a paint tray, then they arent very big.
I am trying to imagine this in my head, a normal paint tray isnt that big, so you could just dump it out when it gets dirty and put in some clean water. you wouldnt have to use a filter then.
What type of turtles are they? I am assuming Red eared or other type of slider.
Sliders get pretty big, a full grown female will require 75-100 gallons of swimming area.

this is a yellow bellied slider, which is about the size of a typical full grown slider.
The turtles may kill that anole if it gets too close.
Pictures of my turtlesbut if you could answer those questions and i can help out some more.