Because they need to recyclate some specific kind of valuable vitamin created by bacteria from rabbit stomach, I think, so they need to pass two times by the gut the same material. In any case what rabbits ruminate does not apply to Tilapia, is a different kind of animal.

If Tilapia arent feeded, they will eat algae and smaller Tilapia. Small tilapia fry can survive eating microbes and algae that grow in organic residues for a while, but this is a different question. Giving cattle the minimum to just survive is not the same as farming cattle.