It should also be noted that fish don't necessarily always float when they die. ... In these cases, the fish will often sink, at which point decomposition will begin. That said, the decomposition process can sometimes result in enough gas being produced and trapped inside the fish to cause it to float to the surface.

Why didn't my fish float when it died?

Most fish are slightly denser than water, so sink immediately after death. However, like a drowned human, they become more buoyant over time as bacterial decomposition produces gases inside the body. Usually, enough gas builds up in body cavities to make the corpse float, like an inflated balloon.

How do you tell if a fish is going to die?