You just cant let go can you 
Just tried it again this morning. Tap water at 32, streaky as hell.
Sorry cant help you,maybe its as others have said and it depends
what is in the low tds,birmingham water comes from wales lovely
and soft but got no idea what sediments etc are in it,all i know is
i get very few problems if any until the tds gets high which is around
a 100ppm in the height of summer if its very dry and then i adjust what i do.