I have one 3 inch comet goldfish in a 20 gallon long tank with 2 filters on it totaling 300 gph of filtration. I do know his tank is too small, I just didn't know that when I got him. I am currently trying to upgrade his tank or find someone who can provide him with 100+ gallons to swim. I currently do 50% water changes once a week. Is this good or should I be doing larger or more frequent water changes?