
hey Spiffy'! .... geez, you can't get a break huh,

... OK, seedlings this young won't have defc. issues, the cotyledons have enough building blocks for them for about a week or so, depending on medium nutrient load,... It's fairly common for folks to add very weak nutes to the final moistening of the coco as well,.. Since you're in coco, adding at least Ca-Mg is mandatory to maintain that cation buffering i was telling you about,.... recall, coco is a Ca hog, Mg to a lesser extent,... this is the cation exchange capacity deal you may have read about concerning coco,... But this issue looks like the coco may not be well rinsed, prep'ed or just not aged enough by the makers,.. coco has a shitload of native Na and K in normally, so it needs to be aged, rinsed, and then cation buffered/rinsed to purge the Na and excess K, replaced by Ca and Mg,.. this is why coco hogs it, a strong binding affiliation (stronger charge),.. so much so that it'll keep the Ca from the plant until sated, then the "buffering" (not pH in this case) of these cations will release and rebind readily,.. straight water can upset this balance... Aaaanyway, if the coco isn't tuned up well, excess Na and sometimes, K can leach out and cause these burn-like symptoms,...
I've not worked with coco myself yet (not that good for outdoors/photo's IMO),.... so I'm not that hip on what the good brick-type brands are,... have you looked into the coco section here? ask around there,... otherwise, get the loose bag stuff,.. Canna Coco is excellent, Royal Gold Tupur, Botanicare, Roots organics,...