Fish is very healthy for you and your brain and eating fish everyday is perfectly safe and okay to do so.
However I cannot eat fish everyday even though I love it I just get tired of eating fish everyday or anything else for that matter.
But if you love fish enough to eat it everyday then you certainly can do so without any bad health effects.
The more fish you eat the better off your health will be and you may even prevent the onset of dementia or other brain issues.
The fish is also good for your heart as well and can possibly prevent a heart attack, stroke, heart disease etc.
I eat fish at least twice per week which is good enough for me but if I wanted to eat fish more often then I could do so.
You will not get mercury poisoning from eating fish everyday as you would have to eat a whole lot of fish per day to get mercury poisoning.