Sometimes flies can get into our toilets, despite the filters, lids, fans and seals. It happens! If you should get flies there, or if you suspect some have been in your toilet a sprinkling of diatomaceous earth may help. This won’t kill the flies, but gets rid of any larvae present in the solids. This white powder seems pretty innocuous and can actually be eaten (check it out online, if interested), but it kills any larvae who come into contact with it.
We sell a 275 gram tin of diatomaceous earth and this can last you ages.