could be a number of things.. but without the details, it's very hard to diagnose. can you provide ph, nutes, dosage, using an air stone?, lights and distance from plants, etc.

are the leaves curling up? sounds like older leaves are affected right?

nitrogen, phosphorous, or magnesium could be your problem, and it sounds like it could be a deficiency of one or more. are the veins on the leaves the same color as the leaves, or purple as well, or staying green? do you have pictures? until you get details out and get a response back, I'd change out your water completely, fresh nutes @ ph of 5.9, and make sure your EC is not too high or too low.