Why should you not eat milk with fish meat ?

After eating food, it goes into our digestive system. There it is digested with digestive juices. One type of digestive juice is required for the digestion of fish and meat, while another type of digestive juice is needed for the digestion of milk. 

Eating both foods at the same time requires two types of digestion. So any kind of food is not digested properly. That's why they don't eat fish, meat and milk. Of course, people's food tastes have changed over time. They are consuming milk or milk products with Amish. But it is harmful to health.