This is nice, i love the idea, but its not very clear how many oxygen you have, how many your fishes consume, etc...
I did't felt any consequance of my actions, i just placed fishes and nothing change. The atmoshpere slowly get better but it does't feel like there is a balance to keep between oxygen, food, etc...
So, to conclude on a positive note: good concept ! Continue working on it and it will become better !