jack keep a close eye on your ph. It may well be rising in response to the stress of the heat or something. It is important to keep where the plants are down to the 5.8-6.0 level....along there. I just lost some brand new seedlings I put in my setup and feel I could have changed that if I had paid more attention to my ph's quick rising. It can be sign of tissue destruction....burn....etc...

Seedlings are frigging wimps at that age.