FIshes have many cells, that is, they are multi cellular organisms. They are also eukaryotes, because their cells have true nucleus and they possess membrane bound organelle in their cells.
Fishes maintain homeostasis because it is the only means by which they can keep their internal environment constant. There are different types of fish, some live in salt water and some live in fresh water. Both type of fishes have to regulate their internal environments in order to maintain the biochemical reactions in their cells which sustain life.