In addition to water parameters are you adding any water treatments or fertilizers. Also what food are you feeding your fish?
Fish normally only eat aquatic algae, plants, or other aquatic insects and animals. They don't consume food that grows on dry land. Some common human foods are known to be toxic to some land animals. Chocolate, onion, garlic, sugar, coffee are some that come to mind.
If you are feeding them something that doesn't naturally occur in an aquatic environment, I would recommend discontinuing using it. Also don't assume a food is safe it they eat it. They don't know what is safe and toxic and sometimes it toxic foods may test good.