Help please. Yellowish leaves with brown tips

Hi have recently started my first grow and things were going good until just after 3 weeks when some of the leaves started to turn slightly yellow and then the tips started to turn brown from the bottom of the plant upwards. Since then a few more leave have started to show the same signs

Am growing in a light compost have not used any nutrients just plain water which i have been monitoring the ph of. Am growing under a 250cfl with 2 fans providing air one in and one out

Any help would be much appreciated

kinda looks light heat stress or light burn. whats the temperature of the grow box

temps are 75-85 the leaves which are burn't are no where near the light they are the ones on the bottom

why aren't the plants potted in the center.

one is slightly off centre my bad. But the picture makes it look worse than it is