Question

Which would have a better chance of becoming a fossil a fish that died and settles to the ocean floor or a mouse that dies on the ground in the forest

Answer 1

I would say that a fish that died on the ocean floor would become a better fossil, because archaeologists nowadays have discovered many fossil that were preserved quite well in ancient oceans. 

Answer 2

The fish. Fossils must be quickly buried in an environment free of oxygen where nothing will disturb them. I.e, the ocean floor. The mouse will be open to the elements for ages before plant matter begins to cover it so will have less of a chance.