I'm building a DIY hydroponic rail system that is growing lettuce and I have a resoivor that has a high quality air pump with 4 air stones and a pump.

After about 8 weeks of what seemed like a successful system, I lifted my letuce pods and noticed the roots were turning brown which I suspected was root rot.

Since I have air stones in my reservior, I do not understand how root rot would happen.

At the beginning I noticed I had put too much nutrients in my reservior (900-1000 ppm) for lettuce, but tried to reduce that by adding mroe water over time. Since the lettuce seemed to be doing okay, I did not worry about it. Is it possible this is the cause of my problem or do I need to place my air stones in the rails closer to the plants?