HELP! She's growing slow and yellow, now curling

[Catacamalaka]

This is my first grow, I'll give as much info as I can. This is a northern lights feminized seed from nirvana. I planted her in a jiffy pellet on the 13th so this is 18 days from seed, I put her in a large dunkin donuts cup with coco and perlite when the first set of serrated leaves came out. I was watering with just ph tap water with no runoff until the first set of serrated leaves began to yellow at the tips, then I added 1/4 strength canna A and B with full strength Cal Mag from GH. Last night I watered until about 30% runoff with a change from 5.5 to 6.3 in ph. Kit came with ppm meter so I measured it at 345 to start and the runoff was 427. I was afraid I overwatered but she looks about the same. The new growth looked yellow since they started and now the edges of the bigger leaf are curling and darker than the rest of it. Growing in a 2x2x4 with an HLG 65 V2 4000k led with the equivalent to 150W metal halide at 12 inches away atm( I was scared I was burning it). My temps are steady at around 76F light on and as low as 69F lights off with a humidity of 47-52%. I could use some advice as this is my first attempt. Thanks in advanced!

 

[hairyman]

Do you have drainage holes in your cups? They need to dry out between waterings. Also, I have had problems with using tap water. It's really hard here and was I think causing nutrient lockout. Using R/O water now with Happy Frog soil and no problems. I do not usually feed until they get several sets of leaves but I use soil. Coco might be different. Read the coco relevant parts of the GWE website for expert advice on that.