Steve, the epsom is worth a shot. Should green things up if that's the problem. I'll do that this week and post results back as soon as I see any.
Doris, no air getting to the roots would make total sense if they grew too thickly in the reservoir. It seems that, with S/H growing, the prudent thing to do then would be to unpot every six months to check things out and clean things up, then up-pot to make sure the roots aren't getting too crowded in there and have a chance to keep getting oxygen.
That must have been an incredible let-down after things were going so well. Three hundred plants--I can feel that pain!