I love the concept of trying to reach "critical mass" of a population, and see a lot of potential in this.
Definitely needs a lot more polish though, menus between levels and an actual "game end" state would be a big step in the right direction. Also a little bit more in-game guidance on how to use the chemicals would be very helpful.
Another awesome mechanic would be some other factor that changes population growth (might be a bit whacky, but imagine throwing in basic genetics, so you want the tadpoles with better genetics to be the one's that reproduce the most).
Overall great concept and a very good effort, keep up the good work. :)