Dead fish and decomposing plants should be removed immediately.
If the tank is not cleaned regularly or if the filtration is not proper, the bioload accumulates in the tank. Soon, algae spread over the tank due to the food available. Brown bacteria also begin to cover the tank surfaces.
These bacteria and algae require oxygen to survive. They absorb the oxygen in the water which deprives the fish of oxygen.
One of the first symptoms of excess bioload is the fish coming up to the surface to breathe.
Bioload which is not removed will result in increase in ammonia and nitrates which will eventually kill the fish. A foul smell emanating from the tank is also indicative of excess bioload.