How many days do you guys go before you you have to change your water? Ive got a 30 gallon tank an aerator rated for 100 gallons, and a filter system rated for 70 gallons. This past summer I had 20 bluegills , 12 big goldfish , and 10 dozen minnows in there, and I had to change the water once every 3 days. Next season , plan on getting a 150 gallon stock tank, and getting another aerator and filter system. I really dont want to have to keep buying filters either, so I need to figure out a way to make a filter that I can just wash out or something. What do you guys think?