Fish is fine, as long as you're mindful of the heavy metal content. Tuna and some fishes are limited to 3x a week as per current guidelines, whereas Fish like Salmon and Tilapia are less restricted as per guidelines.
Two such things I know that can help chelate, (get rid of), heavy metals, include Tryptophan the amino Acid and Vitamin C, so your intake of those would to some extent affect the bodies ability to rid itself of such metals.
As for other foods, Vegetarian sources of protein include.
Rice, Peas, Mushrooms, Lentils, Flax, Fish, Eggs, Cheese, Peanuts, Nuts, (to a small extent), Cows Milk, Goats Milk, Soya Milk, Rice Milk, Almond Milk, Hemp products and Soy products including Soybeans, Tofu, Tempeh, Edemame and Nattokinase.
Hemp is around 25% Protein and Soybeans 33%, the most concentrated protein source in nature I believe and Eggs are around 18% Protein.
Very small amounts exist in things like Cereals and Broccoli, but don't be relient on them or Nuts for goodly amounts of protein, as you will get modest but not goodly amounts.
In the case of Nuts, maybe you would from a cup, but you're talking roughly 700+ calories from a cup, owing to all the Omega Fat content.
I would say Quorn should be good for Protein, as is TVP, (Textured Vegetable Protein), which by the way is gross, but to each their own
So you do actually have plenty of options
