Fish is 'good for the brain' for people consuming too little ALA (alpha-linolenic acid) and too much LA (linoleic acid), which is the vast majority of humans. Fish is especially good in the case of extremely large amounts of LA because it contains preformed EPA and DHA, which normally would be produced from ALA but the excess amounts of LA prevent this from happening due to both ALA and LA requiring the same enzymes.
I don't believe that fish would be beneficial for people eating 80/10/10 however as even the lower fat ones (3% fat instead of 10% for example) tend to get over recommended amounts of ALA (0.5g to 3g are the ones I've seen) and a balanced ratio of LA as opposed to the 1:15+ ratio which is fairly standard for the rest of population.
There are only 2 essential fats: alpha-linolenic acid and linoleic acid, all the other ones can be synthesised by the body hence why people can gain some fat eating large amounts of carbs. When considering if you get enough fat or not look at the amounts of those two essential ones, not the non-essential ones as eating 100g of saturated fat isn't going to provide your body with anything other than energy.
I apologise for the lack of references but all that I've written is basically common knowledge within the area of human nutrition and you shouldn't have any trouble confirming it.
The brain is made up of mostly fat, but that doesn't mean it needs large amounts of fat to maintain itself. That would only be true if it was constantly degenerating and rebuilding itself.
Instead, the brain needs only small amounts of the right fat, which can be found in fruits and vegetables.
Personally I think I go against the grain a bit here by airing on the side of caution when it comes to fat amount, I eat an ounce of walnuts each day (the only overt fats I eat) which are almost entirely made up of essential fats with a very limited amount of saturated and other non-essential fats which along with my fruit and greens on a 3000kcal diet tends to put me at around 7g ALA (omega3) and 12g LA (omega6). This amount of essential fatty acids is obtainable while still staying in single digit percentages when it comes to calories from fat, but also easily matches even the higher dietary recommendations for EFAs by nutritional bodies.
I'm not certain that these amounts are necessary and there are indeed plenty of sources suggesting that lower levels are sufficient and that the original study used to establish LA requirements was flawed but I'd rather be safe than sorry and sit on the fence until more research is done and it's fully understood.
hey check out the videos under the topic fish on nutritionfacts: