Hmmm... :)
Very nice and a bit refreshing game.
Things needed for improving the game:
Upgrades for the tank ended too quickly. It would be nice if the tank would have even more levels of upgrades.
However, weapon upgrades should be adjusted accordingly. For an instance: In some circumstances, double-fire rate gun or triple gun is better than that big gun (especially against basic shooters). Big gun is a bit too unbalanced, except maybe for that biggest enemy. I think that the final weapon would be a combination of rate-fire, spread fire while the big projectile is launchd for every 5th or so click.
Bubbles should be also a bit bigger.
A little HUD would be nice which would include.
1. A minimap-like thing (showing enemy locations inside a bubble) and something that indicates your current position on the map.
2. A scoreboard.
3. Something that shows how much bubbles do you need until the next weapon upgrade.
P.S. How many different kinds of enemies are in the game in total? I counted over 8.