they do that, over time you end up with one species of zooplankton usually. One tends to dominate to the detriment of the other population. There is some scientific law/theory that explains the best in closed systems, but it escapes my mind at the moment.
The thing about "TONS" on a hobbyist level is that it is only "some" on a commercial level.
I think to cull 300-400 a week from the micros, I would need a couple thousand at all times. That is a pretty small broodtsock actually. The wild snail colonies we manage, (I prefer to call it managing rather than collecting because the majority of the work we do focuses on keeping the numbers up...) are all over 100k, (not that I count them, we do square yard or on some species square feet densities and then times by total sq feet/yards etc...) The dwarf cerith colony is over 2 million, and we were forced to start another colony of them, they just breed too fast and at some point can starve themselves out.