Sick autoflower seedlings - urgent help needed please

Hi all, im growing 3 auto flower seedlings and there not looking too good. There in a dr90 under a 250w hps, I am growing in 15l air pots using a mixture of coco, perlite & some clay pebbles, the seedlings are about 10 days old now.
The watering schedule has been whenever there feeling light and 2" dry ontop, if so i would then water to runoff with ph balanced tap water (pH 5.. I did begin in using root tonic @ 1ml/l but recently changed to plain tap water when problems occurred.
The temperatures where a little high, pushing 30C but with some recent alterations its down to 25-27C, the light is a good 20+inches away.
I will upload pictures asap, the plants at the moment are looking pale and droopy, one seedling has lost 1 of its first serrated leaves due to it drying and crisping off.
Please help me ive ran out of ideas to resolve the issue.

T
The second picture is the worst looking seedling, the stems far more pale than the other 2.

I know they're small but you could give them some calmag.

ph 5.0 is really low.

Sorry i meant 5.8pH. would you reccommend feeding a 1/5 strength 0.25ml/l of cal/mag ?

Cheers for input