steve wrote:More filtering does not make the water more healthy, there is a limit to what that can accomplish.
Pretty much.
We all know turtles produce lots of poop. I'm betting there's lots of pee too, but I can't see that.
Filters deal with the nitrogen cycle ammonia (and urea) -> nitrite -> nitrate. That's it. It ends at nitrate unless you have some really special setups. We need water changes to remove that.
Filtration, particularly with carbon, can remove some other stuff and make the water look clear. But there are likely other chemicals building in addition to the nitrates. Again, that's where water changes come in.
Not that there's anything wrong with overfiltation. It will help with water clarity and can respond more quickly to sudden changes like a dead fish trapped under a rock. But even with overfiltation, you still need water changes.