Fishy!

Fishy! is a little Flash game that kind of exemplifies one of the basic rules of nature, eat or be eaten. You play the role of a little fish who has to eat his way up the food chain without getting eaten himself.

Gameplay that’s as simple as it can possibly get

Fishy! is played with just the arrow keys because you’ll only need to swim around and point your fish at a smaller one which can be eaten. The goal of the game is to eat as many fish as possible and grow to get the biggest score there is.
You start off with a fairly small fish and you’ll have to swim around carefully until you encounter some that can be eaten by you. Every fish is randomly generated so there’s no ‘fish appearance pattern’ to learn. You’ll just have to be patient until smaller fish come swimming your way.

Apparently in this game, fish can eat with their tails too

The fish in Fishy!, not just the one you control but all of them, seem to have evolved to a point where they are capable of eating with their tails and fins. It would have made more sense and the game a bit harder if you could only consume fish with your mouth but it seems that just touching them with your tail results in a kill.
However, the bad part is that bigger fish can do this to you too so you need to be extra careful when swimming around, one little touch and you’re dead. This aspect makes Fishy! relatively hard to play because it takes a while to get used to the sensitivity of the controls and the amount of fish that swim by.

A tough little game that can be amusing when there’s nothing else to play

To wrap it up, if you really don’t have anything else to do, or play, Fishy! might be a good option. It’s casual harmless fun that’s also becomes a bit challenging if you take it seriously.