So guy's you use pure water do you? Best not tell the custys you don't 
You don't use pure water you use filtered water, even at zero parts per million the water will still contain dissolved
solids.
I prefer to tell my customers I use deionised water it sounds more technical and complicated.
Sorry I thought the whole point was to use a tds meter as it reads the disolved solids in the water via conductivity so when it reads 000 ppm if there are still disolved solids in the water then why bother taking a reading at all and just use tap water? 
Everything is relative.
If you want to go the extra mile to attain "pure water", get yourself an Ionics parts per billion system.