If the plants are at the end of their cycle, the edge of the fan leaves and some of the other leaves will show necropsy. That's normal. If this happens at any other time during growth or flowering, it's most likely a chemical problem. Look at ppm of fertilizer. Check ph of water and correct to 6.8. Hydroponic solutions should be changed every 7 days for best performance. More fertilizer than is required is NOT better. After the plants take their nutrients all that is left is salt. Salt is not good for the plants. To change solutions more often than required is better as opposed to not changing it often enough, which is bad. Follow directions on nutrients/fertilizers to the letter. Again, more is not better. It will burn the plants and could kill them. Get yourself some testing equipment. Most important is a PH checker. Next a PPM stick for nutrients. If you know exactly how much water is in your tank/reservior you can determine exactly how much nutrient you will need. If you don't know exactly, get a EC/nutrient stick or a meter that will tell you how much salt is in your solution.
