This article is intended to give you some idea of what to do if your plants do not seem to be as healthy as they could be. The problem with an article like this is it's rather like reading a medical book containing symptoms of various diseases; you seem to have them all!
Bolting of lettuce Lettuce is a cool weather crop and will bolt if it is too hot or if it doesn’t get enough light from germination to maturity. Give it plenty of light early on, but keep in a cool shady place after partial maturity. Use 40% shade cloth.
Damping off This disease is also called root rot, although damping off applies more to seeds and root rot to plants. It is a fungus disease caused by a variety of fungi. It strikes seeds, which will turn mildewy and fail to grow. It may be due to the fact that your unit is in a place that keeps it too damp, dark and cool. In plants, the roots turn brown and rot. One answer is an all-purpose fungicide.
Drooping leaves Check to see that your plants are getting enough water. Check that the pump is operating correctly. If these are all right it is possible that your nutrient concentration is too high. Flush the system and run on plain water for a couple of days, then use the nutrient again. Make sure that there is a good circulation of air around the unit and that it is not too hot.
Limp Lettuce When they are grown in the sun, lettuce leaves are sometimes too limp to serve. They will crisp up nicely for your salad if you wash them in cold water, shake gently and put them in a plastic bag in the refrigerator for an hour before using.
Overfeeding This is a common mistake, and burns the plants. It is safer to run nutrient on the lean side. If the plants get too little nutrient, you will know they need more because the veins of their leaves will turn yellow or pale. Just add a pinch of nutrient powder and they’ll turn green again almost overnight.
Rain overflow If a heavy shower should cause your reservoir to overflow, it is best to throw away the solution and prepare a fresh one. Light rain will have little effect on the solution.
Spindly growth This is caused by insufficient light.
Sunburn White, bleached-looking patches appear on the leaves. Plants such as tomatoes and cucumbers need a 30% shade cloth, while lettuces do best with 40% shade cloth. Tip burn The tips or margins turn brown. This is caused by overfeeding. Flush the system and make up a new batch of nutrient solution.
Yellow leaves When a vegetables is nearing the end of its fruitful life, its older leaves will usually turn yellow and die. This is normal. When younger leaves yellow or turn a distinctly lighter green than older leaves, try adding a pinch of nutrient. Wait a few days and they’ll turn green again. If not, add a little more.