Leaves dark green and curling up

One of my babies (there are 4 all using the same res) have what looks like Mg dificient (leaves are dark green but curling up at the edges) there is no signs of yellowing yet but it looks like the "praying for Magnesium" pics i've seen here and there. I would post a pic but my damn camera needs batterys lol.

Heres some of my variables tho
Air Temp: 75 F
Res Temp 72 F
Lighting: 400 watt MH 4000 kelvin 18 inches away
pH: 5.8
PPM ??? need a TDS meter hehe
Nutes: GH Lucas formula 0/5/10 full strength
Plant(s) age: 5 weeks, maybe 6
Apparent maturity: root are starting to go into res (more every day), plants are vegging at maybe 1/2"-1" or so a day

Mg deficiency isn't as common as some think. Tough to tell without pics though.

Are you using distilled or tap? If you really think it's mag, you can add a little epsom salt - start with 1/4 tsp per gallon. If you're using tap, and depending on where you live, you probably don't need mg.

there are a bunch of good products for those using distilled. One is called "Cal-Mag" and it may even be General Hydro product.