Show HN: An aquarium game with a real nitrogen cycle

What are the Nitrogen sinks? I expect Nitrogen to enter as part of the protein in food, but then it will be eaten and release as urea or something and accumulate. How does it disappear?

Small feature request: To feed them I have to click the "feed" button and click somewhere in the tank. I expected that clicking again somewhere in the tank will release more food, but I have to click again the "feed" button. It's not obvious when the "feed" mode is locked and when not.