Praise be to Allaah.
It was not narrated that the Prophet (peace and blessings of Allaah be upon him) forbade drinking milk at the same time as eating fish. Since no such thing has been proven, then we should refer to the doctors concerning this issue. If they say that eating these things together is harmful, then they should not be eaten together, rather they should be eaten separately. But if there is no harm in eating them together, then you may do so if you wish. It may be that eating them together is harmful for some people, because of individual sensitivities or allergies, or the nature of their bodies, whilst it is not harmful to others. In conclusion, this matter should be referred to the experts. And Allaah knows best.
Sheikh Muhammed Salih Al-Munajjid