Indigenous people eat fish of all exotic and plain variety as a major part of their diets, not just modern-day Scandinavians and world-wide ocean tribes. If there was any danger that you are consuming too much E, D and B Vits (all fishes possess them, oily or not), then would also have direct health-ailments on all human populations, even if not at all diabetic. Substituting Fish with say Red meats to balance 4s FBG should not significantly affect the results. However, on a personal note, it is essential that some red meats are eaten twice weekly, precisely because Iron content in Greens (vegetables), although rich in many vitamins, absorbs differently in the blood-stream altogether. A young person 16-25 can get away with a vegetarian diet for instance without risking their health - was vegetarian myself for many years, yet by age 32 was severely anaemic without even knowing this. Not that this will happen to everyone, yet certainly can contribute to overall physical health as we mature.
To be really sure I would say speak to an experienced dietitian, though there does exist a lot of individual bias towards healthy eating from one practitioner to the next depending on what school of thought they have studied and most passionate about. My recent dietitian was hospital-based and her opinions agreed with mine, not all will have the same opinions as yourself.