I use oyster shells to control ph, cleated Iron should be available at any garden or nurseries. Use sparingly. also how old is the system yellow leaves can also be caused by nitrogen decencies maxi crop makes a seaweed fertilizer with cleated iron. I supplement this when I'm starting a new system before the bacteria builds up
My pH is around 7 in all my systems. The systens are less than 4 months old. The ammonia is less than 1 ppm. there are Nitrites in the water. What kind of stores supplies the maxicrop. I live on Oahu Hawaii. Others have suggested the Nitrogen problem. I will try to find Maxicrop or something like it.