What are you fertilizing with? What kind of potting medium are you using?

I'm currently fertilizing with Sustane 4-6-4, and plan on spraying fish imulsion once I figure out what's happening with my plants. Medium for this container is Down to Earth's Pro-organic mix, crushed black cinder that is 1/2 and smaller sized, and compost mixed in (I'm learning about Al's 5-1-1 soilless mix, but I'm in Hawaii, and a lot of mainland's easy-to-find resources will cost me a fortune here, so still trying to figure things out). I'm wondering what the brown spots might be because it's happening on other pepper plants that are younger/in different mediums. Is this rust?

bump