Buy a decent potting mix that has 0 fertilizers in it. (one for tomatoes is fine if a Cannabis specific one is not available), mix in 20% or so of Perlite. Purchase some decent cannabis nutrients lke Canna or whatever (they all work), use at 1/2 the recommended dosages and grow away. Don't over think it, your in soil.
Get rid of the bloody bark your trying to kill the seedling with.