the guy at the shop is a dumb fuck, just like budsmoker said all that will happen is it will slow the filter down a bit and you will have to change out the sediment prefilter more often but regardless you need it bad. my old house the water was 280 ppm and i thought it was bad, now at my new place the tap water is 35-40 ppm with a ph of almost 8 and the plants hate it so my new grow is with bottled water so far until i can figure out where to mount a ro filter, but yeah at my old place i used a britta filter just to cheap out but it brought my nasty water down to around 60 ppm and more than a half point in ph so in the mean time i would get one of those, as for your flowering plants give them distilled water from here on out for best results, i mean shit even if you flush with that shit water you are still giving them more shit than most people do nutes