Hi, I am from Malaysia too. I am no expert but I know some of the information that you needed. So, bear with me for my long essay.   

Omega 3
Yes, the advice to consume more fish which is high in Omega 3 does help baby and young kids’ brain development. It also helps adults’ brain in a way of preventing dementia (also some other brain related disease), heart disease and some other illnesses. However, the brain beneficial Omega 3 actually refers to 2 out of 3 types of Omega 3 which is called docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). The third types are called alpha-linolenic acid (ALA). This is why the milk powder for infants and children are fortified with DHA and EPA. Noted that food that is fortified with DHA & EPA and Omega 3 supplements are no match for natural sources.

The sources which contain DHA & EPA are definitely come from meat or animal based. The animal based that have highest DHA & EPA are the marine animal (includes seals & whales) especially those oily fish which live at colder region because they need to keep a lot of fat in their body in order to thrive in cold region. Freshwater fish that live in colder region also have high amount of DHA & EPA compare to our tropical freshwater fish but lower compare to marine/saltwater fish that live in cold region. Other animal based that contain DHA & EPA but in lower content includes grass-fed cow & chicken egg.

The alpha-linolenic acid (ALA) is usually found in plant such as walnut, flaxseed, chia seeds, soy, leafy greens and etc. Plant based contain little to none at all of DHA & EPA. As such, I don’t recommend to eat more plant in your search for beneficial DHA & EPA. I didn’t mean that ALA is bad for your health. ALA is also beneficial to human health in other ways and our body need ALA to transform it into DHA & EPA. However, this transformation of ALA into DHA & EPA in animals and human are not efficient.

All animal includes fish and human are only able to convert ALA into tiny amount of DHA & EPA. However, oily fish able to contain the highest amount of DHA & EPA by accumulation. It mean that ALA which contain in aquatic plant such as phytoplankton, algae, seaweed and etc are being eaten by small fish that convert ALA to DHA & EPA which lead to being eaten by medium fish which not only taken in the DHA & EPA from the small fish but also convert some ALA into DHA & EPA in its own body and so on until it reach the top of the food chain, the apex predator in the ocean such as tuna, salmon, mackerel and etc. Fish that live in cold water would save a lot of this ALA, DHA & EPA in its own body as part of its body fat while tropical fish didn’t save much.

Mercury
Mercury itself is a naturally occurring element that is present throughout the environment and in plants and animals but human industrial activity (such as coal-fired electricity generation, smelting and the incineration of waste) boost up the amount of airborne mercury which eventually finds its way into lakes, rivers and the ocean, where it is gobbled up by unsuspecting fish and other aquatic. This is one of the reasons that mercury contents in animal especially in fish are raising.

For your information, mercury usually refers to as inorganic form while the mercury in fish are the more toxic organic form called methylmercury but I will just refers it as mercury. It is true that freshwater actually contain more mercury than saltwater. However, it doesn't mean that freshwater fish have higher level of mercury than saltwater fish. Because in freshwater, mercury tends to latch onto decayed plant and animal matter, where sunlight can easily break it down. However in saltwater, mercury latches onto chloride (salt), where sunlight has less of an effect on those bonds. By being longer in saltwater, it increases the chance that a fish will ingest more of the mercury.

Once mercury gets into the marine food chain, it “bioaccumulates” in the larger predators in a same way as how Omega 3 accumulates. That’s why larger fish are generally riskier to eat than smaller ones. Fish not only accumulate mercury from consuming smaller fish. All fish absorb mercury from the water that passes through their gills. The longer the fish lives, the more mercury it will bio-accumulate. As such, species of fish that live in saltwater where mercury does not break down easily by the sunlight, long-lived and/or high on the food chain contain higher concentrations of mercury than others.


Solutions / Guides
As I mentioned earlier, mercury exist in our environment, plants and animals. It is just a matter of the level of mercury concentrations. As such, you could always choose fish which is low in mercury but high in Omega 3. Unfortunately for us, Malaysia doesn’t really that strict or efficient in providing an easy access and detailed list of mercury content in local fish (especially freshwater fish). The list below is from USA release by their U.S. Food and Drug Administration (FDA). Please be noted that since the list is from USA, the type of fish listed might be different/different name from what we know in Malaysia and/or Asia. (e.g. lobster - they refers to saltwater variety, the freshwater variety are called crayfish/crawfish in USA, bass such as sea bass, black bass are not our Asian Sea Bass/Barramundi/Kim Bak Lor).

Highest Mercury Fish
Pregnant and nursing women and young children should avoid fish high in mercury, according to the U.S. Food and Drug Administration (FDA). High mercury fish to steer clear from include:
• Tilefish
• King mackerel
• Shark
• Swordfish
• Marlin
• Orange roughy
• Ahi Tuna
• Bigeye tuna

High Mercury Fish
While not the highest in mercury, you’ll still have to limit these mercury-containing fish to three 6-ounce (170g) servings (or less) per month.
• Spanish mackerel
• Gulf mackerel
• Albacore tuna
• Yellowfin tuna
• Sea bass
• Bluefish
• Grouper

Low to Moderate Mercury Fish
These fish contain low to moderate amounts of mercury, so it’s generally safe to eat up to six 6-ounce (170g) servings each month.
• Striped bass
• Black bass
• Saltwater bass
• Mahi Mahi
• Lobster
• Fresh water perch
• Sea trout
• Canned light tuna
• Skipjack tuna
• Cod
• Halibut
• Trout
• Whitefish

Lowest Mercury Fish
If you’re concerned about ingesting mercury from fish, choose fish lowest in mercury — especially if you’re pregnant or nursing. Low-mercury fish are excellent choices for children as well. The FDA says you can enjoy up to 12 ounces (340g) of low-mercury fish per week if you’re pregnant or nursing.
• Salmon
• Herring
• Haddock
• Anchovies
• Butterfish
• Crab
• Clam
• Crawfish
• Catfish
• Croaker
• Flounder
• Chub mackerel
• Atlantic mackerel
• Oysters
• Pollock
• Perch
• Mullet
• Mussel
• Rainbow trout
• Freshwater trout
• Sardines
• Shrimp
• Scallops
• Squid
• Sole
• Tilapia

Best for Pregnant Women
Not all low-mercury fish are high in omega 3 fatty acids, which are fats needed for proper brain development in babies and children — and important for brain health in adults. Therefore, the Environmental Working Group (EWG) provides a list of the best overall fish options for pregnant women because they’re low in mercury and good source of omega 3s:
• Sardines
• Herring
• Oysters
• Pollock
• Salmon
• Mussel
• Atlantic mackerel
• Rainbow trout

While the threat of mercury contents in fish are real and should be concerned. It may be made too extreme by the trends/hype. It is because mercury in fish has always been exists and human have ate it since ancient time. It is just made worse by human activity. While lesser mercury contents in those days are one of the reasons for the ancient people being not affected by mercury, there is another reason. That reason is selenium. There are some studies about how selenium prevents the bad effects of mercury on human body. And all fish contains selenium. As such, by eating fish which contain low mercury and moderate to high selenium, maybe even add some food which is high in selenium such as chicken egg, shiitake mushroom, white mushroom, chicken, brown rice, soy bean and etc into your diets, it should help you to be able to reap the benefits of Omega 3 while reduce the risk of mercury poisoning.

If you're still worry about the mercury in fish, maybe you should check your own mercury level first. After all, fish are not the only living beings that is affected. The next thing you know, you or your family already have high level of mercury contents in the body. Go for a blood mercury test. Women with a high blood mercury level who are planning to start a family may decide to postpone pregnancy for a few months until that level drops.

As for the name of edible fish, I assume fish which is locally available in Malaysia? For that, I will post later .