I've been vegetarian for 10+ years. For the past year or so have added fish once every week or two, because I was already taking fish oil capsules for the DHA and EPA and figured it would be healthier to eat fresh fish and get all the nutrients available. Dr. Rhonda Patrick is a research scientist with an excellent website and podcast series on nutrition, diet, health, longevity called Found My Fitness.
http://foundmyfitness.com/

If you're gaining weight: eat less, exercise more