Yellowish new leaves on TMS

[JayC]

I have seven TMS plants that have looked great for the last few weeks. This morning I noticed that a lot of the new leaves are yellow. All the older foliage looks nice and green. I did recently water them. It had been two weeks and they pots were super light but the plants had not shown any signs of being thrusty but the soil was very dry and I was away for a few days and wanted them to be OK. I'll post some pictures when I get home in a few hours. My other concern is that they have outgrown my light setup and only a small portion of the plants now fit under the fixtures. Could this be the reason? I used my normal well water that I always use to water so not sure whats going on there. Any ideas....I know its difficult without pictures.