I'm only 12 inches from the tops of mine as well, running a 400 HPS with a basic alluminum reflector.

Like Tiny said, it's probably just a little over fertilization.

Flush it with 6.5 PH balanced water, and continue to water with plain water until signs of fresh healthy growth. Then hit 'er with some half strength nutes and see how she reacts.

As long as you have proper airflow, you should NOT be getting burnt spots on the leaves. Heat stress will make the leaves look like tacos or canoes before burning/cooking occurs.

besafe:jointsmile:

be