Orb Farm

They eat waste from algae, but they seems to devour it too quickly, and may even spur more algae growth. The balance and influences seems to be a bit of a mystery, despite the documentation.

If you create an isolated section where the top 1/3rd is algae and there's a tiny bit of bacteria at the bottom it's trivially self sustaining, until CO2 runs out. So these can be sort of "Oxygen generators" to power the rest of your system.

This seems to form the overall basis of the system since during the day the O2 rises and during the night the O2 falls. Further, at night the daphnea awaken to eat the algea, and the fish can eat the daphnea. In theory this should balance but in my experience daphnea are absolute monsters and need to be tightly controlled.

I create small stable environments and then a larger middle environment. The small stable environments regulate O2 and CO2 with algea and bacteria.

The middle environment is then some sand, grass, and fish. There's also algea, a very very small number of daphnea, and bacteria.

The middle is the chaotic one but if your 'regulators' are doing their jobs you can keep things stable. Make sure there's some shade so that your fish can hide from the sun (a thing?) and so far I think my system is ~indefinitely stable. I have 4 or 5 fish at a given time, although I have 7 at the moment but I suspect that's a peak value. edit: OK, I have 9 fish actually, so maybe I can sustain more than I had though. I may need a bit more O2 output for this many, but there's plenty of grass for them to eat.