thats not nute burn mate, bb light mix hardly contains any fert, ive used it loads. look at the stem, its well purple, thats a P def im sure. seedlings use quite alot of P and K in the first weeks developing a rootball, iv had that with the light mix loads and a light feed of bb bloom fert sorted it out, bb light mix really is literally just for germed seeds and rooting cuts, pot up into a bigger pot of bb all mix and it will sort itself out from the extra fert in the all mix, if you are in uk save yourself some pennies and get some westlands west+ multi purpose soil with added john innes at ya local b&q, it pisses over bb and canna soils at less than half the price.