from what I understand, its high concentrations of salt build up that kills off the bacteria. I use drip dwc at no more than 900 ppm and rinse(flush) my hydroton at 200 ppm once a month to prevent accumulation of salts(drip ring helps keep hydroton clear of build up, as I use 10 inch net cups) we are using microbes to prevent/cure pythium, not to break down organic compounds. keep anything organic out of your dwc, and all should be well.
as far as valves go, those are the only ones ive used. cheap and easy installation. just make sure you use the right size hole for the grommet for a snug fit.