Not unusual to see a bit of burn on seedlings--they recover and are very hardy. One leaf loss is ok. Had one with undeveloped Cotyledons and is thriving. With the coco you will have better control. I use bagged Canna Coco and pretreat it. First, run 5.5 ph water to just run off. Let it sit for a few days.
Add some dry Great White (probably would not use - but - have some left from soil and it is expensive stuff) to center of pot a few inches down and mix a bit. Then irrigate with about 1/5 to 1/4 lowest nute level recommended. In my case Canna a and b with a little root stimulant.
Once the seeds germ--use spray bottle with ph water and root stimulant and spray around area where will drop the seed. Then just use the spray bottle to mist and keep soil damp (mostly for RO). Misting doesn't seem to trouble the seedlings with the inclusion of very little root stim. Spraying keeps me away from over or under watering. So, for the first week or two the plant will seek out the water and not get root bound. The top soil does need to be moist--all the time. Coco is notorious for drying out and seeds suffer.
After three weeks or so begin regular watering with low nute and work up. RO water is best for coco with cal/mag added. Use cal/mag every watering. In addition, as Rosenthal advises--add Cal/Mag (and Si--if use use it) then PH. Next add all nutes and ph again. This gives the coco a buffer. Generally go with 6.0 ph. Seems to be working. These techniques are from Canna Coco, Rosenthal and the spray is just a way of keeping my hands off the seedlings and providing RO and cool down---I too am in a hot and low RO environment.
Cheers------------go over to the coco section and start your journal. Loads of info. over there.
