...before getting new fish? We had 4 perfectly healthy guppies, then had to get a new one from Petsmart. That had some kind of illness and before that died within a couple of days, it also managed to infect all of the other fish in the tank. So now we have a tank full of water with some kind of germ/parasite/bacteria in it and we need to get the tank ready for the replacement fish. After dumping the current water, what else do we need to clean the tank and get rid of any remaining germ either on the gravel or on the rocks/plastic plants/filter/heater, etc. Do we boil the gravel and rocks or use some kind of fish-safe cleaning agent? Thanks for the help.