Aquar.io is a great multiplayer IO fish aquarium game that you can play online and for free on Silvergames.com. Start by controlling one single fish and move around eating all kinds of stuff and finding other fish to join your army. Once you control a whole lot of units, you will be strong enough to go hunt other players.
Be careful with the dangerous jellyfish that float all over the field and avoid crossing larger groups or you might end up losing your units or, worst case, your life. Become the leader of the aquarium and have fun with Aquar IO!