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 )