Oftentimes magnesium and sulfur is what the peppers really need, although calcium and other nutrients are important though as well. When my leaves started to yellow, I added some epsom salts which has both, and they greened right up and recovered quickly..
I grow naturally, but I personally stay away from kelp and fish emulsion because of heavy metal contamination. I have seen countless studies showing that most kelp and fish have too many toxic metals such as mercury; and recently kelp was found to have elevated levels of uranium. The plants uptake those toxic minerals into the plant, and thus into the fruiting bodies. With the amount of pollution out there already, I personally prefer not to take any chances. If one could find a source of liquid kelp grown away from the oceans and tested for heavy metals such as Maine Coast sea vegetables, I might consider buying some, but I have yet to find one trustworthy. I have used it before though with excellent results when added in addition to other minerals and ferts.
It should be noted that many plants can take toxic metals and still grow quite well, unlike us who can develop chronic conditions and diseases with low levels accumulated over periods of time.
