Yep, magnesium deficiency. An alternative solution is to put 10ml of epsom salts (get from any good pharmacy) in a pint of water and foliar feed once or twice, not forgetting to spray with pure water the following day to wash the leaves and aid absorbtion. always do this just before lights off or if outdoor just before the Sun sets so as to avoid burning the plant. Some growers as a matter of course spray their plants with this solution a day or so before initiating flower so as to avoid problems like hook bud. After all, provention is better than cure.