I'd have thought a balanced fertiliser with trace elements would have been good for a container plant. It doesn't take much to change things drastically in something the size of a pot. I haven't used chelated iron so don't know about dosages. Maybe someone with direct experience will chip in.

I decided to go with a very unscientific "tiny pinch" to a gallon jug of water. It was still enough to tint the water pink. So I gave a couple ounces of the pink water to each of my plants and followed up with regular water. Hopefully this is a dilute enough amount not to cause anything terrible. They haven't spontaneously combusted anyway. My best guess is that the plants were being watered with our very alkaline tap water before I bought them, and that led to some iron deficiency.