Shouldn't imagine hardness will be a problem if you're using rain/tank water Bernie... unless you're going to be stocking crustascea ... marron, yabbies, prawns...
And you can address carbonate hardness issues through the buffering affect of utilising shell grit, Calcium Bicarbonate... etc.... the sort of stuff that many of us use to correct pH drift anyway.
Epson salts is another way to address "hardness"..... but generally it's not a huge factor in AP systems anyway....
As to the liner.... any particular reason that it has to be that particular brand??
Welcome, by the way... good to have you here.
