What is wrong with my babies, first grow :(? pictures

if your ph was at 7.8 - 8.0, then you might shock them with the adjusted ph of 5.5. I would gradually lower the ph over a few weeks to avoid problems.

If I had to say what I think it is from some of the things that have happen to me, I would say that you might be using your nutrients too soon and it could be a ph problem because the jiffy pots might be retaining to much of the nutrients. What I would do is flush with 5.0 ph balanced water with no nutrients for about two weeks, cut off the dead and/or dying leafs and see if that will help, at least that’s what I did to my plants and everything turned around. I hope this helps but again this is just my opinion.

Also you may want to switch to a soilless mix like "Roots Organic" coco fiber mix.

Well they are going into flowering now, im doing short veg and theyve been vegging for 15 days now. I did not use any nutrients prior to the leaves yellowing, i was thinking it might be a nitrogen def

Try using 1/2 strength of nutrients for about a week and then gradually increase the nutrients.

Use only purified water for the first 4 nodes and kinda work the baby into some light nutes. The soil could be burning them so just flush it and let the soil dry out, but not alot, you don't want to over dry them.