Not heat stress.

Let the medium dry between waterings. Water slowly till the point of runoff. Don't water again till soil is almost dry. Obviously this is different for each comtainer, each medium and each growroom condition. It's something you'll have to get used to. Once on a feeding schedule, same thing. Water till runoff, let dry-out. If still a few days before next feeding, add some fresh, properly ph'd water. If having problems with medium staying too wet, go get some perlite to mix-in during the next transplant.

Keep ingoing ph near 6.8 or 6.9 as FFOF tends to be on the low side of acceptable range. Whomever mentioned lowering water ph to 6.0 is wrong. Acceptable ph for peat-based mediums is 6.3 to 6.8ish. Your ph will drop further over time as the buffers in the soil fade, so keeping it a tad on the high end of the range is acceptable and warranted.

Keep your ph consistent to rule-out ph fluctuation as a cause of plant stress. Quality demands consistency.

Unless the medium is pre-fertilized, it's getting near time to start them on 1/2 strength nutes. Which are you going to use?