Please help me? Ugh

This is my first grow and I am messing it up pretty good. I've made almost every mistake you can think of so far. Read on man. It's bad.

My seeds germinated fine. I transplanted them into a soil that I mixed myself with organic potting soil from home depot, pearlite, and peat. First I didn't have the lights low enough and they stretched. Moved to a few inches away from the 900w LED. Made the classic rookie mistake of overwatering. Got fungus gnats. Got rid of them using food grade Diatomaceous earth. Repotted them deeper to encourage more root growth and let medium dry out a bit. Temp is 77 with the use of a heater and RH is 60%. Running a box fan on low and a programmable humidifier. They are in a grow room that I created, which has no exhaust/ ventilation until this weekend. I'm trying to PH my water to 5.5, but it ends up at about 5.7.

My questions are: The bottom leaves are yellowing and I have very slow growth. What caused it and can I fix it? Seeds are expensive and I can't afford this chit. If it's the soil (I don't have a soil PH tester), can I change it out or is it too late for that? I'm not going to stress them out more than I have and I need to save them. I'm truly medical and can't afford to buy at the dispensary any more.
The pic I'm attaching is from a plant germ'd on 1/15, planted on 1/17. Today is 1/29. This is the only plant that is yellowing, but they all have stunted growth. Please tell me they are salvageable.
 
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And seeds are way over priced for as easy as they can make them...IDK where you are located,but clones maybe a better solution..
 
I'm legal but we're just don't have clones yet. Or seeds. I still had to order them.
Its alright nothing to stress about it's all learning and taken the mistakes u made this whole grow and apply to next grow shit I have had three full runs and still messing up but I just keep it in my MGB in my memory lol and MGB is marijuana grow bank lol
 
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Im sure someone will have better ideas,but if me..if get some coco/perlite or plain potting soil/perlite carefully replant.. water with a lite calmag,plain pH water for few days..talk nicely to them...watch them improve the transplant as needed...not saying that's absolutely correct...just a way to simple it down..they are so easy to burn at that size..
 
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Im guessing that when mixing in the added peat additional lime was needed to buffer the ph back up into the 6’s. The organic potting soil was likely buffered but the amount of lime it had is dilluted now.

Im sorry i dont know how to advise how much line to mix in as top dress but can be done. Go easy is always good advice.

Also maybe using tap water would be better than ro for the calcium content to help keep the ph up in the soil.

The seedlings look like their soil is acidic for sure.
 
Im sure someone will have better ideas,but if me..if get some coco/perlite or plain potting soil/perlite carefully replant.. water with a lite calmag,plain pH water for few days..talk nicely to them...watch them improve the transplant as needed...not saying that's absolutely correct...just a way to simple it down..they are so easy to burn at that size..
Well right now they're being called little beyotches. I wanted to change out the soil with coco, but I'm afraid of killing them. I looked at the price of ffog and was sure there was no way i could afford it. I just found those blocks on Amazon yesterday. I didn't even know they existed before.
 
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