Yellow leaves and other symptoms

I have some plants that are about 3 weeks old about 4 - 5 inchs tall and the leaves are turning yellow/light green curling down and have brown spots appearing. Im guessing its cause i may have used nutes to early. only at half strantgh.

Im now just feeding 5.5ph water no nutes for last two days but they are not getting better. temp is fine light is close but only using a floursent at the min.

Over fertilisation or not?