How is the PH and what are you using for fertilizer and how much if any? Get a ph tester if you don’t have one first. My thoughts are lack of magnesium caused by low PH. You could try giving them some epsom salt to boost the magnesium. Just use 1 tsp epsom per gallon of water. It’s not going to hurt anything either way to try this. Cut back to just water and epsom for a week and look for changes. This is just what I would do first so let’s hope we get a couple more suggestions.