I've been potting up lots of plants recently and was alarmed at the high TDS readings coming from the trays. After replacing the water the TDS had creeped up again. When I tested a bucket of RO water that had only long fibered sphagnum moss rinsed in it, it came out to over 500 ppm. What could be the cause of such high TDS readings? The moss in question is a 5 kilo bale of 5 star Chilean that appears to be high quality, at least visually.