i second, third, or whatever the suggestions of more tanks and/or increased capacities of the tanks as well as the ability to buy multiples - that is more than one set of eggs at a time, more than one medicine, (my one dissatisfaction with the game is having to surrender one medicine for the other so that i am paying for both but can only use 1 at a time).. a population counter for the tanks and a way to distinguish pregnant fishes..
a way to add decorative plants, features, and rearrange what is there - such as the temp regulator being next to the air in tank 2...
lighting, more varied sounds - the sound of the fish jumping is too frequent
it is interesting reading Arthur's comment regarding solving the puzzle - the fish are fun and relaxing to watch and breed that it is easy to loose sight of the 'puzzle' aspect! it is addicting!!