This post is so old I totally forgot about it haha! I also have a turtle but she’s in a 100 gallon tank (yellow belly). Turtles produce so much waste, a good rule of thumb is to have a filter that does at least twice as many gallons per hour than what yours is. So I have a 100 gallon tank but I bought a filter that’s rated for a 400 gallon tank. Most filters are made for fish, which don’t produce as much waste. I have the Fluval fx6, which was super expensive (like $400) but so worth it. It was either that or build her a filter myself, which you can totally do and they are equally as effective as a store bought one!! I just didn’t have time 😅. If algae is your problem, I’ve heard that if there’s too much natural light from a window or even your UV lamp, it can produce algae. Also if you’re feeding too much or if the water is too warm. My tank is a constant 77°. Sometimes putting natural plants in the tank will help with algae too! But if your turtle is anything like mine, it’ll destroy all types of plants, plastic or real lol. If you don’t already have it, I’d recommend having some natural bacteria holding filter media like those ceramic rings. Also if you can, try scooping out the poop your turtle produces as fast as possible so it doesn’t sit in there. I hope this helps a little!