Lots of twists and turns to this.
To start with, the desired active ingredient in fish oil is called Omega-3. Omega-3 is known to have anti-inflammatory properties, and works best in a balance with Omega-6, which most people get more than enough of in their diet. Importantly, Omega-6 has a mild inflammatory property.
So if you take a fish oil supplement, you want one with minimal Omega-6.
In turn, Omega-3 has two ingredients, called EPA and DHA. Of these two, DHA is the one that provides the greatest benefits. So much so that it is now being sold as a separate supplement. (Not to be confused with DHEA, which is a different supplement.)
Importantly, many types of fish oil are known to be high in urea, so if a person has a predisposition to gout, caused by too much urea in the blood, they should not take fish oil.
Otherwise, from a dietary perspective, fats and oils, by themselves, do not contribute much to weight gain, and may even result in weight loss, *unless* they are combined with carbohydrates. The two together most definitely are the recipe for weight gain.
So if you consume more fish oil, you should consume far less carbohydrates. And of those carbohydrates you consume, they should be far fewer simple carbohydrates, like sugars, and more like complex carbohydrates, long-chain carbohydrates that are harder to digest.
Do not be deceived by sugar substitutes, especially sugar alcohols, that do not affect the blood sugar levels because they are digested far down the digestive tract. They are still carbohydrates, and will still work with fats and oils to cause weight gain.
High fructose sugars are regarded as worse in this regard than ordinary sucrose, because they are more readily converted into fat.
One of the contributing factors to weight gain that may be helped by fish oil, are clusters of cells found in the fat, and proportionately more in the fat of obese people, called MAST cells. MAST cells produce histamines when irritated, resulting in something like an allergic reaction, but one that encourages fat cell growth and retention.
The DHA in Omega-3 in fish oil may help settle down irritated MAST cells, making it easier to lose weight.
Huh? What does this mean? What shouldn't we be deceived about? If the body doesn't recognize a sugar substitute as a carbohydrate, how will it be digested "far down the digestive tract?"
High fructose sugars are regarded as worse in this regard than ordinary sucrose, because they are more readily converted into fat.
Regarded worse by whom, nutritional illiterates? This statement also makes no sense. What else in your post should we be suspect of?