With some polish this would be a pretty good short time waster, but there are a few things you should fix. The game should show how many bullets you have(or at least if you have any vs none). There should be a warning where the tanks spawn: It's quite impossible to avoid them when they spawn right next to you. Also, sometimes bullets spawn right at the edge of the map, making them hard to see.
If you continue developing this game, some kind of high score list would also be a good idea, probably.