Big leaves are dying, turning yellow and gray

I have a prob with my moms and clones. The older fan leaves are turning yellow and gray and basically rotting on the plant. I am doing hydro under T5 lighting. Even the clones with roots are starting to rot. My water is PH'd and I am using Ionic grow nutes, only a few hundred PPM. I have a lot of gnats that I have noticed lately but no other bugs.

Any ideas?

I am thinking it might be from temp getting too cold causing K deficiency. I havent had a heater on and its prob getting below 50 F.

Potassium - Too much sodium (Na) displaces K, causing a K deficiency. Sources of high salinity are: baking soda (sodium bicarbonate "pH-up"), too much manure, and the use of water-softening filters (which should not be used). If the problem is Na, flush the soil. K can get locked up from too much Ca or ammonium nitrogen, and possibly cold weather.