It's difficult to diagnose your problem without pictures to reference but I'll give it a try. Since your plants were doing fine prior to this problem, I must assume something has changed. You mentioned that you change the nutes every other week. Do you adjust your nutes between these changes? It is necessary to add nutes between reservoir changes to replace the nutes the plants absorbed. Even if the TDS of your reservoir isn't changing, you can rest assure that the plants are consuming your nutrients.

Are you getting plenty of Oxygen? An absolute must! Are your reservoir temperatures too high? If so that will prevent the water from holding oxygen. If your PH gets out of range, this will lock out nutrients as though they weren't there at all. A common mistake for new growers is to add more nutrients when they see signs of deficiency instead of lowering the PH making the already present nutrients available.

In this case, because you're seeing a widespread reaction, I would recommend an immediate reservoir change. The size of the grow and the environment they're in will dictate how much nutes to use. It would help us if you posted some pics or described the size of the plants and the environment they're in so that we could more precisely diagnose your problem.