definately not the airstome
could be problem with EC ,,,,but not aparently so looking at the tips of the leaves gives no indication of over ferts,,,,
what about the ph?? a high PH may have this affect on the plant, as at the higher range of ph the plants cant convert the nitrogen efficiently and lighteneing or darkening of the leaves and stunted growth are the results,,,,, check out your PH range and maintain between 5,5 and 5,7 . you may need to try and restabalize the ph in the arlita, so comply bytrying a more acid sollution for a few days,,,,at a ph of like 5.5. but get a digital PH and EC meter! as soon as possible,,
a hydro grower without the equipment is like a camel without a hump!....
catch ya
good luck..