Been over this many times , all toilet additives  become food  for the bacteria(and other single cell organisms) lurking in the environment and  as this bacteria live, multiply and eventually die a  film builds  up on the inside of the flush tank  , just waiting for more food ( additive). If you or THO insist on using an additive in the flush tank make sure you regularly drain it down and use something like Puriclean/Domestos or Milton regularly to  try and kill most of the  little darlings ( you will never kill 100% of them unless you use 60% hydrogen peroxide) and drain the tank  down afterwards. Your service engineer , when  he has to change the pump will thank you!. There is also some empirical evidence from members on various forums that the additives can seize up the flush pump spindle. As Mr Plodd  says :-  fresh tap water is best , after all  that  is what is used in domestic toilets and comes with about 400 ppm of active chlorine ( which is the active  element in most bleaches )