Fish are mobs added by the Mo'Creatures mod that consists of many animals, like Sharks. When you kill them, there's a chance of them dropping a fish egg. When you get a fish egg, you can hold it in your hand and right click on it, and it will spawn a tiny egg mob (you can pick it up by walking over it).
Any kind of fish will drop an egg
An egg (no matter what kind of fish it came from) will spawn a random fish, so that means you can have a large variety of fish!
Fish cannot be mated, you can only get more through eggs, but if you wait then more will spawn naturally in a large body of water
If at any point an egg is taken out of water, it will die.
If an egg hatches and it is too close to a wall, it will suffocate in the wall.
If you have a nearby cat (tamed or untamed) it will jump in the tank and kill the fish, then if you don't act fast, the cat will also drown.