Hi guys , I’ve recently started a new tank. And plan to start filling tp tank, after not using the rodi unit for 3 years, I changed the filter and starter running it, 1st batch of water was 18ppm. After 30 mins of running it is not at 6 ppm. Is 6 ppm acceptable for sps tank? And what’s the reason for the drop of ppm as I run more water?