Go to my grow journal here on my blog spot.

LED Logger

On the right of the main page you will see some links. Look for plants problems and disease.

In that they will tll you the problem and the solution. Looks like a Mg issue to me too.

It might be that there is too little magnesium or the Mg has been locked out.

Try using 1 tablespoon Epsom salts to 1 gallon of water, see how the plant reacts to the solution. Is the water very hard in your area ? This could be the problem too.