Most fish when they die do not sink to the bottom, they float to the top. Just look in your aquarium sometime. This is usually caused by the fish having gas in its swim bladder. This gas is used, when it is alive, to make its job of rising or sinking easier. Fish, such as sharks, that don’t have a swim bladder can fall to the bottom on death.
However, by the time most dead fish are “ripe” enough to hit the bottom, their state of decay eliminates any possibility they have of ever forming a fossil. Once on the bottom fish are usually eaten by scavengers or totally rotted by bacteria. Next time you are out scuba diving or even watching an underwater TV program, look carefully for the number of dead fish on the bottom of the sea. You will usually have to look for a long time.
If you examine most fish fossils you will find that the only way you can match in the laboratory what you see in the fossil, is to bury the fish alive, so that it dies in a trapped situation in a state of shock, suddenly and catastrophically.
The existence of large deposits of well preserved fish fossils points to only one thing—rapid catastrophe. The common concept of evolutionary textbooks on Geology, that fossils were formed slowly over vast periods of time, is pure imagination.