What alternatives are there to eating fin fish?

Take fish oil capsules, ideally enough to get 1,500-3,000 mg total omega 3s per day.

A tbsp. of chia seeds contains ALA omega 3s. Leave in water for 5 minutes and swallow. Unlike most other seeds you may swallow them whole. They don’t replace EPA, DHA and DPA omega 3s found in fish, but they are still helpful and may partly reduce your total omega 3 needs. So you can take a little less fish oil capsules. At least 10% less, and probably 20-40% less.

These are all essential fatty acids meaning they are fatty materials that your body can’t make on its own. They are important materials of the brain and heart. You can get enough ALA from plants to barely make enough of the other 3 to survive, but it isn’t ideal. The only other practical source is expensive and smelly algae supplements that taste worse than fish, and so often contain flavoring and fillers.

There is nothing essential or necessary in fish.
In fact, I could make a very good argument that you are better off not eating any fish.

The one thing that maybe should be a concern is Omega 3s. And there is little to none scientific/medical research showing that Omega 3s are even necessary.

If you are really worried about Omega 3s they are easy enough to supplement. Don't take fish oil capsules tho. If you are worried about omega 3s you can add a tbsp of ground flaxseed to one of your meals. Or more expensive (but still cheaper than fish) is flaxseed oil. its even available in capsules.

Although ALA gets converted to the other forms of Omega 3 in your body, it is a slow process. DHA and EPA are also available as a supplement and there are vegan versions as well.

Fish is a good source of protein but there are many other sources of protein. Even vegetarian and vegan ones.

Fish is also a good source of toxins and heavy metals so those are some good reasons to avoid fish.

If only fish has the health benefits you want, as you say in your last sentence, then you're saying there isn't anything else that has the same benefits. Your question then doesn't make any sense. But if you want other sources of omega 3 then try seaweed, algae, or meat from grass-fed animals.