Well, it could be a lot of things. The main causes are too much light (especially if it’s coming from a window) or high nitrates. For green water, at least.
How big is the tank? I’m willing to bet it’s a either too small or you’re not doing enough water changes- or both. The rule is 10 gallons of water per inch of shell length so if it’s a full grown male he needs a 75gal and a female needs a 100gal.
Canister filters are great, but all tanks need water changes. Turtles are high waste producers so water changes might need to be weekly. Do you have a test kit? If you can, buy a liquid test kit (API sells a good one). Test for ammonia, nitrites, and nitrates.
You’ll want them to read like this:
- Ammonia (NH3/NH4) 0.0
- Nitrites (NO2) 0.0
- Nitrates (NO3) 20ppm or lower.
If you read any ammonia or nitrites that’s a problem. Ammonia doesn’t actually cloudy your water, though. What’s creating the ammonia is one culprit. Excess waste in the tank could be to blame, and the solution is the same. Water changes! Depending on tank size this could mean 25% per week or larger.
So my questions for you are: How big is the tank? How long has it been set up with the turtle in it? How often do you do water changes, and how much water do you take out at a time?
For reference, this is my turtle tank. It’s a 75gal and only has my “little” ~4.5-5” guy in it. I clean it 50% once a month because it has a 70gal canister filter and about 60 gallons of water in it.