Yes selenium will neutralise the mercury and we don’t know for sure the type of mercury in fish is harmful it is just the general dogma on the topic. There are untold “facts” told by the medical industry that aren’t necessarily true in reality. Many of us here have likely eaten excessive amounts of fish alongside amalgam fillings (which I believe are harmful to health) and will not suffer the negative effects of mercury toxicity until our late years, if ever. The topic is overblown imo and very few people have life threatening ailments or symptoms from “mercury toxicity”.
One thing you can do is to chelate the metals, which is dangerous and taxing on your organs in itself.
Something I like to do is to overcompensate minerals that mercury would displace in the body; such as magnesium and zinc in a cyclic fashion.
Dietary fat can also work to absorb or work to bind heavy metals, your fat soluble organs or bodyfat itself is where mercury is generally absorbed and stored.
The body tends to lock away heavy metals within itself to protect from further damage should they displace.
Either way, we’re all poisoned with heavy metals to some degree and mind over matter and a positive attitude should always be our first practice when it comes to health. I know many people who give in to their ill health mentally and create a life with even more suffering of their own creation. The mind is very powerful. I ate literal tons of canned tuna alongside 5 amalgam mercury fillings which leaked for decades, I am still a fully functioning athlete and as for the mental decline I’ve had over the years - how do I know to attribute it to heavy metals? Nobody knows for sure. We like to find issues so we can pay for a solution to our problems in this life, most of the time we even go looking for problems.
I wouldn’t worry about the mercury in fish honestly, I’d worry more about cancer of the butthole from the baby wipes I use to wipe my ass with, than that.