One of the most popular myth in our Pakistani society is that Taking milk with fish is harmful and it can cause skin disease. Due to this popular myth most of Pakistanis don't drink milk or its products before or after eating fish.
The fact is that its not true and there is no harm of taking milk with fish. this myth became popular in society because it was a hindu belief and Pakistan and India where united before partition. that is why this belief also became common here.
Medical science also rejects this myth that using fish and milk together is harmful. infect there also some dishes in many countries which are made by fish and milk together.