Its a gimic. The fish are not harmed...there is plenty of water left once the toilet is flushed.
I know what u are saying..but really..they would be fine. All that happens is that when the bog is flushed, about 1 third of the water from the cistern is removed, to be replaced with freash, clean water. I can tell u know....i have seen some aquariums that have been filthy...much worse for the little buggers that live in those.