I have a 20 gal high aquarium. I has 1 large fancy goldfish, 1 small Pleco, and now 11 zebra danios. In another thread, it said to do water changes at a certain %, unless you have goldfish. I asked about "with goldfish", and was referred to start my own thread. How much % water change should I do in this tank? Currently I do 25% change each week, with an occasional 50% change. Any recomendations?