Fish Random Movement

So I am making a fish eating game, where you start out as a small fish and eat more to get bigger. The game is WAY in the early stages. However, I want to get some help on the movement of the npc fish in the game. Right now their movement is OK at best. Any tips on making it more realistic?

http://www.flowlab.io/game/play/392857

Arrow keys to move.

like agar.io?

Yeah similar! agar.io is like a clone of the old school fish eating flash games. Want to make my fish move more believable. Not sure if i should just continue adding layers of random number movement or if there was possibly a better way.

maybe add more animation(like a wake behind the fish, and maybe make the tail wave?)
Its all up to you