As has been said before, low-oxygen conditions limit scavenging, so a dead fish can be slowly buried.
And low-oxygen environments are common in lakes, and in the ocean:
(sorry, I have links to all this, but I can't post links yet).
Anoxic settings have been broadly categorized into four types: (1) lakes whose bottom waters are anoxic as the result of water column stratification; (2) silled marine basins in which freshwater influx (either rainfall or river inflow) exceeds evaporation; (3) oceanic upwelling zones where deep, nutrient-rich water flows to the surface adjacent to a continental land mass; and (4) open ocean anoxia, which typically develops at middepths (200-1000 m) at the edges of some ocean basins (Demaison and Moore 1980).
(From here: earthscape.org/r3/whelan/whelan20.html)
You will notice that at the anoxic zone is often present only at depth. This means that fish do just fine in the shallow water, but when they die they sink into the anoxic zone, where they are protected from scavengers, and can be slowly buried by fine-grained sediment.
Alot of "fish eating fish" fossils have been found in the Green River Formation. The GRF is a famous varved lake deposit, definitely NOT produced by "the flood".
Of course, the fish could also have died and been buried rapidly, perhaps by a submarine landslide (which are very common, even today).
But there's nothing at all to indicate that that fish was buried by a biblical flood. There are plenty of ordinary, well-documented modern mechanisms capable of creating that fossil.