that leaf drooping is bothering me, it's been a while, but i thought overheating will make the leaves bend in a V shape, not droop. what kind of soil are you using? it appears to be peat and virmiculite, but i would bet either you bought the pre ferted stuff that will burn a seedling, or more likely, it doesn't have ferts and since peet has no nutrients at all, they could be getting a little nitrogen deficient, wich would explain the lower leaves yellowing, if i had my guess i would say your overwatering though, do you let the soil fully dry out before you rewater?