Stop at cotyledon stage, true leaves do not grow

there's a definite benefit to providing some level of nutes even early on, but you may have gone overboard. Did any fertilizer drip on the plant? something you should avoid too. You might be giving too much light this early.. nodes are quite tight despite initial stretch when it sprouted. as long as you didn't get some cut-rate coco, it's probably okay.. condense bricks need a lot of work before you can use them, but the loose 70/30 premix stuff with perlite should be ready to use. not all brands are equal in this regard. untreated coco can be poisonous - lethal or at least slow them down for a long time. i don't think that's the case here, though. I usually have a 1 EC charge of nutes in my substrate to start. that's never caused a problem. ppl use potting soil that comes fairly hot all the time with no problems... it's only a problem if you made it too hot, in regard to fertilizer concentration. first irrigation before you sow the seed or transfer the paper towel result, can be water-only.. but your irrigations after sprout should definitely be at least a 1 EC charge of nutes and quickly ramp up to 1,3 to 1,5 EC. Religiously get 10% runoff with coco/soilless context and any formula changes go into effect quickly. 10% runoff also maintains concentration, avoids buildup. if religious about it, it avoids numerous problems.