In some ways, this game reminds me of Pac-Man.
Pac-man was a very simple game. The controls were simple, the graphics were appropriate, there was no real ending, and it was insanely addicting. The same holds true for this version of Bubble Tank.
The name "Bubble Tank" is quite appropriate since once you upgrade to the level with force fields, you're pretty unstoppable. Interestingly, it seems the bubble tank can't really be destroyed. I tried getting all my upgrades destroyed and when I was back to most basic version of the tank and got hit, I was simply got moved to a new, safer bubble. Cute.
Finally, I'm very impressed that it's only about 300kb. Small is beautiful.
If this game needs room for improvement, it would be the sounds. I'd like an option to turn it off as it gets repetitive. Next would be an ending, or a storyline, but I think those changes will both increase the file size and remove the game's charming qualities.
Overall, excellent effort. Well done!