Best Fish For a Keto Diet | Keto Fish Benefits | The Simple Approach- Thomas DeLauer.

Sardines:

Sardines provide one of the highest sources of essential omega-3 fatty acids and have 740 mg of DHA and 450 mg of EPA in a serving - more than 1000mg of omega 3’s combined (an average 3.75 oz can contains roughly 23g of protein). This is a nutrient that's a key component of a healthy thyroid gland and it reduces inflammation by inhibiting NF-kB and its activation of interleukin-6 and TNF-alpha production. 
Selenium is also essential to the proper functioning of glutathione peroxidase - glutathione is 100% dependent on having a selenium atom at its core for proper function

Mackerel: 

Mackerel is also a fattier type of fish – 100 g of Atlantic mackerel (the most common type, and safest in terms of mercury) contain approx. 14 g of fat, 0 g carbs, 18.6 g protein and comes at 205 calories - more than 1,000mg of omega 3’s. It’s very high in vitamin D, and is one of the few food sources of it – 100 g contain 360 IU of it, or 90% of the daily recommended value
T cells rely on vitamin D in order to activate and they would remain dormant, 'naïve' to the possibility of threat if vitamin D is lacking in the blood. When a T cell is exposed to a foreign pathogen, it extends a signaling device (vitamin D receptor) with which it searches for vitamin D

Salmon - Butcher Box (Wild Sockeye Salmon):

Be sure to choose wild salmon over farmed salmon- Wild salmon eats other organisms found in its natural environment, farmed salmon is given a processed high-fat feed in order to produce larger fish. As such, farmed salmon has a high concentration of contaminants - including polychlorinated biphenyls (PCBs), dioxins, and several chlorinated pesticides. One study, published in the journal Science, investigated over 700 salmon samples from around the world and found that on average, the PCB concentrations in farmed salmon were 8x higher than in wild salmon.

Omega 3’s:

3.5-ounce (100-gram) portion of wild salmon contains roughly 2600mg of omega 3’s - a 3.5-ounce serving also contains 22–25 grams of protein. Half a fillet of wild salmon contains 341mg of omega 6’s, whereas half a fillet of farmed salmon contains 1944mg of omega 6’s.

Halibut:

A half fillet (about 159 grams) of dry-heated cooked halibut - Atlantic or Pacific - contains about: 223 calories, 42.4 grams protein, 4.7 grams fat. Despite it’s lower fat content, a half fillet of halibut contains about 1,064 milligrams omega-3 fatty acids vs 60.4 milligrams omega-6 fatty acids.

Trout:

Trout is another fish that is relatively easy to find and is packed with important nutrients - a 100 gram serving of rainbow trout (the most common type) contains 5.4 g fat, 21 g protein, 0 carbs and has 138 calories. It’s a great source of B-complex vitamins, as well as of potassium, magnesium, and selenium - additionally, it’s packed with omega 3 fatty acids with a 100 g of trout contain 986 mg of it
It’s also low in mercury and polychlorinated biphenyls (PCBs).

References:
1) Omega-3 polyunsaturated fatty acids augment the muscle protein anabolic response to hyperaminoacidemia-hyperinsulinemia in healthy young and middle aged men and women. (n.d.). Retrieved from https://www.ncbi.nlm.nih.gov/pmc/arti...
2) Parra D , et al. (n.d.). A diet rich in long chain omega-3 fatty acids modulates satiety in overweight and obese volunteers during weight loss. - PubMed - NCBI. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/1...
3) Fish oil intake induces UCP1 upregulation in brown and white adipose tissue via the sympathetic nervous system. (2015, December 17). Retrieved from https://www.nature.com/articles/srep1...
4) Effects of n-3 Polyunsaturated Fatty Acids (?-3) Supplementation on Some Cardiovascular Risk Factors with a Ketogenic Mediterranean Diet. (n.d.). Retrieved from https://www.ncbi.nlm.nih.gov/pmc/arti...
5) Essential Fatty Acids: Omega 3 and Omega 6 | Ruled Me. (2018, May 1). Retrieved from https://www.ruled.me/omega-3-omega-6-...
6) Using Supplements on Keto: The Top 16 and Why You Need Them. (2018, April 19). Retrieved from https://www.perfectketo.com/using-sup...
7) An Increase in the Omega-6/Omega-3 Fatty Acid Ratio Increases the Risk for Obesity. (n.d.). Retrieved from https://www.ncbi.nlm.nih.gov/pmc/arti...
8) Toxic Effects of Mercury on the Cardiovascular and Central Nervous Systems. (n.d.). Retrieved from https://www.ncbi.nlm.nih.gov/pmc/arti...
9) Methylmercury poisoning: MedlinePlus Medical Encyclopedia. (n.d.). Retrieved from https://medlineplus.gov/ency/article/...