Simple but charming game! Fish are cute :) Taken to the extreme, one could accumulate a ton of cash, so saving up and then spamming fish is a fun strat. Interestingly, oxygen and carbon dioxide face huge swings at high fish populations - meaning even by spamming plants, fish can still die from no oxygen. At high fish population, food takes just seconds to run out and is another limiting factor. The background music eventually stops too.
Clownfish don't live in freshwater, so that was quite funny. I tried to see just how many clownfish could fit in the tank before their death rate was too quick to keep up with, quite a lot...