Keep caustic soda (Sodium hydroxide) away from your glass at all costs - it destroys glass, eats away at it, turns it all nasty and cloudy.
In labs, it's always stored in plastic bottles. If it were to be put in a glass bottle, with a glass stopper the two bits would fuse together.
I don't know if the same things are used for toilet cleaning in different countries - I certainly don't think there's acid in your ordinary Domestos!
There's lots of things which remove limescale - but they're not appropriate for places which have soft water and don't deposit lime. (ie. here in Scotland.)