Hmm well its got me stuffed mate! Superthrive will help with the stress but we still need to work out what the real issue is...
Ok so the pH is good, you're only feeding 1/4 to 1/2 strength nutes, you're only watering every few days...i cant think of what else it could be besides them being hungry. When i grow in soil, the first 2 weeks i feed with nutes once a week and then after that i feed every second watering with nutes and ive never had issues. If you're ok with the possibilty of maybe losing these ones, maybe try feeding with 1/2 strength nutes every 2nd watering. So freshwater, then 3 days later nutes and so on. Sorry man, im really stumped as to what it could be. Did you get some dolomite?