i've been useing gg4 with hard tap water,
at first i could'nt get on with it but after persevering with it for two months now i think its the best window cleaning invention after wfp.
just use it excactly as it says on the tin, don't be tempted to put extra in.....5ml per 1gallon..............
but like i said you've got to give yourself time to get used to working with it.