Is it magnesium problem or something else?

My plant has little yellowish-brownish spots on the older leavs. I have read, that it may be the problem of magnesium. Could anyone tell me, is it really magnesium or something else and what should I do? Thank You!

I don't know why the picture is not show. Picture link is here:
http://public.fotki.com/sipsik744/plant/brownish-yellowish.html

looks more like calcium actually
get a picture of the whole plant with blue light not hps

Looks like nute splash burn, got nutes on the leaves when you watered them from my angle anyway!lol! Peace