I too have been drooling over these dosers, but I can't help fear that these things would work perfectly 99.9% of the time, but when they go bad.... it would be real bad.
Having said that, I have confidence in the fact that I have yet to here that someone has had a catastrophic failure, like having the whole ph up res get dumped. Anyone hear of such a catastrophe?
Those that have the ph Nanny, being as it is a drip feeder, can you see a problem with salt build up eventuating? This makes me think a pump would be necessary for reliability.