White tips on seedlings leaves

i'd wager the growbig dose you gave plus the initial ~1EC charge in promix might be a bit too much for the seedling -- especially if there was no runoff water. Becareful with fertilization early on. What the promix has in it is enough for 7-14 days at least. Then, since soilles you'll be watering with 10-15% runoff waste and that will maintain whatever EC your fertilization is in the substrate. 1.3-1.5EC once you ramp up. Observe and react to plant. Take notes for next time. It will help, but don't expect 100% constency in how things turn out.. ballpark idea only as so many variables can change. Even your regional seasons will impact what you do at times. No big deal.. if not getting worse, just keep irrigating as needed. The early leaves are sometimes fugly, too. If newer growth is flawless, i wouldn't think twice about the cotys or single/three-bladed leaves. soilless-- fertilize every single irrigation -- though maybe not the first. You may need to ramp up over first 2-3 weeks, while some plants can take more sooner. Can use the coty's as guide.. once they start to pale, you know it needs more nutes. If that time period is fairly consistent, you can premeptively start to fertilize around then in future with confidence. Always water to runoff of 10% or more. This will just about guarantee no EC swings in your substrate barring massive microbial problems. That's waste water.. goes down the drain or okay to use on outside plants only. don't let it sit in it. You'll just grow nasty stuff and roots will be drowning in stagnant, high EC water at bottom of pot. Wait for top layer to dry, unless you are trying to do multiple fertigations per day. Learn the weight at that point as it's your best cue to water. You'd need way more perlite for frequent irrigations, though.. Regardless, in future go for a cumulative 50% portion of substrate being perlite or similar amendment for drainage properties - vermiculite, rice hulles, perlite, hydroton, whatever.. though huge and chunky is not best (lookin at you hydroton and the super coarse perlite that is 1/4-1.2" large or large wood chips, lol.. just making roots work 3x harder to go around it). You'll get better growth rates, reduce risks... only downside is you may need to water sooner than before. Do that with a 1.3-1.5 EC and you will not encounter many problems.. if you do they will be slow-moving and easy to be patient while correcting anything. Take notes... until it's familiar. it will speed up learning curve. Find a ppm calculator online.. knowing what you feed in a soilless grow is very useful information... once you dial stuff in you won't bother, but early on it helps illuminate causality.