Curled Leaves on the Peppers

My peppers have been growing well. Hard to tell in the images but all four plants are loaded with peppers. I've had to prop the Jimmy Nardellos up to prevent them from falling over. I don't know what I'm going to do with the Hot Wax peppers this year.

Something I noticed a while back was that all of the leaves are curling at the edges. I haven't figured out exactly what is causing this. There are no bugs on the plants which is a miracle. I think this is the first time I've grown peppers and not had problems with aphids. There are no spider mites to be seen anywhere on the balcony. It must be a nutrient deficiency but I'm not sure which one or why that would be a problem. There are no other signs of problems. No yellow or dead leaves. No problems with fruit set. Nothing. Perfectly healthy plants with.