I clean mine eveytime it gets empty enough to be worth dumping out. It's a 70 gallon tank for a 2 year old and small donkey, so every 4-5 days, I would say. I just dump it, take it out, scrub it out with some Dr. Bronners and a toilet brush, rinse it out, and fill it up. It get really dirty, but only for the last 2 days, and as long as I clean it that often, it's mostly just sludge from whatever hay they drop in, and not algae. I end up feeling guilty, anyway, though.