Iron and potassium deficiency ?

The brown spot could be from rubbing up against the clay pebbles during lights out. The serrated edges look to be curling up slightly. This is a sign of heat/light stress. You might want to raise the light slightly. Your PH is perfect at 5.8. Hydro is different then coco and the range is 5.5-6.5. If you can you want to lower your water temp. It would be better if you could get it down to around 19C as your currently on the edge of where pythium likes to take over. If you cant then make sure you have a lot of bubbles. EC seems high to me for the size of the plants but some strains are heavy eaters. Happy Growing

They look fine to me nothing dreadfully serious. I think they will grow through this and probably take a few days but Im sure they will power through look green enough to me a very good shade of green. that brows spot could be anything and at this stage it would probably do more harm trying to sort it than its worth rather than just letting her get on with it being in coco you have advantage of being able to flush straight away if needed so Id take full advantage of that fact flush it plain ph balanced water for a day if you must do something but personally I would just let it be. Seedlings are very hard to get right especially for the beginner ive been growing a fair while and I still have problems with seedlings sometimes. they are very fragile though its very common and every grower has hadto put up with it at some point if not they are either lying or not been growing very long lol best of luck for the rest of your run bro Im sure youll smash it the best tool we have as growers is patience :) 👊