What would happen if you only eat fish?

See: Eating fish is great, but eating only fish will lead to loss of needed vitamins and nutrients. That's exactly why the diet should consist of vegetables as well.

Unbalanced diet: Eating a fish only meal is a modified high-protein diet low carbohydrate low fiber. This can lead to constipation and other gastrointestinal complications. Try to eat a daily balance diet from all food groups avoiding high fructose corn syrup wherever you can.