Scott'-- My thinking is this is from the heat spell, which can beat up seedlings at times,.. was the RH really high, or low at the time? Also, what soil is this specifically? ...heat, transpiration stress, and perhaps soil that on the edge of being too rich or off pH can cause this temporary tip/margin burning,... not all strains, even pheno's respond the same too, confusing matters more! New growth looks fine, so that's a good sign,... And as always, the in-soil pH is something to monitor often,.. sometimes soil right outta the bag can be off, usually too acidic. Soil is best around mid 6's, or a bit lower,... best tool for this is a soil pH probe, like the Accurate 8...
Your water sounds pretty good, do you have a TDS or EC meter too? Knowing the hardness (amount of pH buffering minerals) in there is part of the deal when adjusting pH for both water alone and nute soln.'s,... If you need a pH down, lemon will do in a pinch, but you can get canning citric acid, cheap, and it's gentler than most other products,... a little goes a long way, so tinker with it some to get familiar... 1/4 tsp/gal will drop pH about 1.5 - 2+ points, depending on your water's hardness! try a tsp/gal of the juice and see how much it changes,.. give it a few minutes after adding to stabilize before taking a reading,...
