You have too many fish in the tank, we usually recommend two goldfish per 10 gallons of water, meaning that you should have a maximum of four fish in your 20 gallon aquarium. The fact that you have so many will not kill them instantly, but it will drastically reduce their lifetime. The most deaths we have in the morning dead fish round in the pet shop is in the feeder comet fish tank because there are too many in the tanks.
There are a few alternatives for you. You can accept that they will live shorter lives and keep them in the current non-optimal environement. You could also redistribute them in new tanks or make a pond for them outdoor if you have the space/time/means when the weather gets hotter. Or you could always find someone that has need of feeder fish and donate a few; although this may seem cruel, we sell comets as feeders only, it is their main breeding reason in pet shops.
I don't recomment you add an algea eater or pleco to your tank as it is already overloaded with fish and one more will worsen the case. The water should be changed every week (maximum every two weeks). As for your filter, the larger amount of gallon the filter is fit for the better, usually we recomment 10 gallons more than your actual tank (if a 20 gallon, a 30 gallon filter or more is best). Water should be at room tempeture.
Last edited by Vinss; February 22nd, 2010 at 11:45 PM.
Reason: forgot to mention some information