Is it SA RTC?
What are your usual parameters for ammonia and nitrite by an API liquid test kit?
What's the nitrate, pH, temp, hardness KH and GH if you measure them?
If all you have is a bubbler-driven sponge filter, I suppose you probably do not have enough filtration, neither enough dissolved oxygen, which is brought about by stirring the tank water.
RTC will not go to the surface to show you that there is not enough oxygen.
It also sounds that you had taken out the old sponge housing your nitrifying bacteria and have installed a new sponge and as a result got the tank uncycled. That will surely stress the fish and make it lose its appetite.