I see you started off by saying they are about 2 weeks old, the slight yellowing might be the fact that they are looking for some N to really take off. Dont know what kind of medium you are using but on my last grow start when my seedlings were 2 weeks old they had already graduated from 1 1/2in RW to 4in RW and were getting 1/4 to 1/2 strength nutes. Air temp not looking bad but I think it was Loc-nar that pounded it into my head that res temps should be between 62-68f to max oxygenation