Dirty Tank Help!!

[Evvan]

I bought my son a Yellow Belly Slider turtle about 9 months ago and I cannot figure out what we are doing wrong to make his tank so dirty! We have the Fluval 206 filter, and it keeps the water crystal clear for about 3 days before the water begins to turn green. The manual says the filter shouldn't need to be cleaned more than 1/month, and yet we barely make it to a week before we can't even see all the way through that tank. We want to get him a friend, but I can't imagine how gross the water would be with 2! Please help me not have to drain/scrub/refill the tank every 4th day! WHAT AM I DOING WRONG?!?! How often do you change the water? Clean the filter? Does river rocks at the bottom help? What about sucker fishes? Feeding inside vs outside the tank? Help!