I've found that raising the temperature a few degrees did help. (Maybe my TLC thinking)

Since they can't move to warmer water in an aquarium, I brought the warmer water to them, so to speak.

As far as abrupt temperature changes go, the peril lies in an abrupt lowering of it. I've never found a gradual 2 degree increase to cause any type of calamity.

As far as the fat theory goes, any fat will coagulate at a certain temperature. What you need to be aware of is, what the temperature of the consumers digestive system is at, will determine if it is digestible.

For instance, take butter, at room temperature it is a solid. If your digestive system ran at room temperature,you would have trouble digesting it, even if it had been changed to a liquid state before consumption. (It would only recoagulate in your system) Luckily, our digestive systems are usually around 98.6 degrees, so no problemo.

So since fish are cold blooded, their surrounding temperature will equal their internal temperature.

So, by increasing their surrounding temperature, you are increasing their ability to digest more protein, and possible fats.  Right?

It might just be me, but a warmer temp and better food seems to work.

I think of it as giving them a blankie, and some chicken soup.