Homebrew PCBs

Along with my multi tank system I acquired some Mega (Seno) 600-021 
immerse tinning solution - it's  rather old but I'll try it anyway !

On the bottle it says to disolve in tap water at 50 deg C - let it
cool 
and use at room temperature. On the Mega site it says to disolve in 
distilled (rather than tap) water. Two questions:

a/ should it be distilled or tap water?

b/ what is the odd chemistry that lets it work at room  temp but not
at 
50 Deg C when increasing the temp normally speeds up chemical
reactions.


TIA

AWEM