My husband has two Yellow Bellied sliders in a tank - we're about to upgrade to a 55gallon tank for them, as they've grown gigantic. However - their tank smells awful, no matter how often we clean it, and the filter never seems good enough. What's the best filter available and does anyone have any advice on the best way to keep them clean and healthy? I confess I know NOTHING about turtles, we've had them for over a year now and they've quadrupled in size and suddenly seem to need a whole lot more attention....!! But they're so cute and so much fun, I want them to carry on growing and looking great!