Guessing it's a bunch of trial and error right now - these systems are pretty hard to predict in general and I'm not aware of a whole lot of research into closed ecological systems. Which is a shame really.
A self-sustaining system like we'd want should come to its own dynamic equilibrium, given a decent enough starting state, which does make the trial and error slightly more feasible, and of course they have to have some idea of the inputs and outputs of the various organisms.
There is some research on these ecosystems [1], and there dynamics [2,3]. The ecosystem is made up of a photosynthetic algae as a producer, a protozoa as a consumer, and a bacteria as a decomposer, and can be stable for over 1,000 days.
Stable for 1000 days is not the same thing as 'will remain clean for 1000 days'. Have this phones a undisclosed problem with overheating that would suggest to add water as heat buffer?
A self-sustaining system like we'd want should come to its own dynamic equilibrium, given a decent enough starting state, which does make the trial and error slightly more feasible, and of course they have to have some idea of the inputs and outputs of the various organisms.