How tall are your plants, and how many nodes?
Does the soil stay quite wet between waterings? It should dry a bit.

Given that set of info, it sounds likely that they are just hungry. Pics would be helpful though. If the tips show scorch, flsuh the plants well with plain water at pH 6.8 and then give a weak feeding of a 'grow' fert with micros, assuming that they have at least 4 sets of adult leaves.