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A few weeks ago I got a new, more powerful filter. In the past, I had to change the water every 2-3 weeks because it began to look a little green. Since I've gotten this new filter, it's been a little over 3 weeks and the water is still completely crystal clear. Do I need to change the water anyway or is it OK because of the filter?

i think you should try to clean the water every 2-3 weeks because nitrates and other invisble stuff could build up and it might be bad for ur turtle, how big's ur tank? if u test the waters parameters and their good after 3-4 weeks you could always just change 20% every 3 weeks!

Most people do partial water changes. I take out about 50-60% of my water every two weeks and refill, even when the water looks clear. Never hurts to do a little scrubbing while I'm in there.

At least do a partial water change.
Even if you add water after evaporation you should do partial changes, because the remaining water will be "hard."

I remove about 30 - 40% of the water in the tank weekly and add extra to compensate for evaporation.