Couple things.
One- your water pH should be around 6.7 if possible. This is not the primary problem you have, though, as far as I can tell...
Next, your seedling soil appears to actually contain ferts. That's classic tipburn. Give a good heavy watering with tap water at pH 6.7 so that plenty comes out the bottom.
Last, if you can't control your temps, at least switch to 24/0 light schedule because you really don't want 'dark' temps higher than 'light' temps. And you'll have to correct this before flowering.

FYI you may see pool pH up/down when you go get the tester- THESE ARE BAD!!! The liquid indicator is fine to use, but you need actual plant up/down because the pool ones are sodium compounds and will kill seedlings.