If your water is 000 it won't be from any tank residue. Half a handful of dust thrown into a tank isn't going to affect the PPM. We talk in PARTS PER MILLION on here. Think about it. Lets say your tank is 300 litres, thats three hundred thousand millilitres of water in there.
My money is on dirt in the air, wind, how close are you to tress. Or the seal on the window. Don't worry too much, its only window cleaning.