If we drink milk after fish what happens ? and what is the composition of both so that they either effect or uneffect the body if mixed together ?

Not a problem: There is no problem with combining milk and fish. Fish is a great source of protein and good omega-3 fatty acids which are helpful for brain function. Milk contains vitamins, minerals and fats. For the most part, combining different foods will not cause you any harm. Make sure that the fish you're eating is not contaminated with mercury -- smaller fish are less likely to have this problem.

Probably nothing: You should be able to consume milk products after fish without a problem, especially if you lactose tolerant. Good luck.