Need help asap with leaves turning yellow with brown spots on my lsd plants?

Hi there,
I ave 8 lsd plants in two tanks and 3 out of the 8 ave yellow leaves with brown spots all over them, Can anyone help me with this problem, ave no pics just now. will try get uploaded tom. they are 21 days in vegetive stage.

Sounds like a magnesium issue but we will need pics to be sure..........


Look anything like this?




Or more like this......
?


One thing for sure, LSD is a heavy feeder......

Never got the links there? will upload pics soon. thanks.

overwatering

Looks like a magnesium deficiency. Heres a link to a good guide to plant problems.
http://www.greenmanspage.com/guides/plant_abuse.html
More info on your grow like: water R/O or tap, nutrients, medium ect would be very helpful in finding a solution.......
Looks like you are growing in rockwool cubes in a flood table?