Krill oil is very good for Omega 3 oils, but it's also environmentally not so good for humans to be using as it's the main source of food for so many endangered and threatened species. Something to think about. Nordic Naturals makes a fine fish oil not made of krill, with a dedicated fishery in clean waters off Scandanavia. New Chapter makes the purest fish oil because they don't purify it, meaning you get more than just the omega 3 oils, but that means you get less of the omega 3 oils and have to take more of them, and they are expensive. Cod liver oil shouldn't be taken regularly -- it can have a laxative effect, and the liver is a detoxification organ, meaning toxins accumulate there. It can be good for skin problems and other things, but for basic omega 3 you don't want liver oil.
The appropriate dose of krill oil depends on several factors such as the user's age, health, and several other conditions. At this time there is not enough scientific information to determine an appropriate range of doses for krill oil. Keep in mind that natural products are not always necessarily safe and dosages can be important. Be sure to follow relevant directions on product labels and consult your pharmacist or physician or other healthcare professional before using. My Father used this twice a day for 3 months as he is suffering from depression, Krill Doctor Professional Brand oil is really effective to fight against these diseases.