Fish flesh is inherently different from the flesh of most walking, slithering, and flying creatures for several reasons.
First of all, cruising through the water doesn’t exactly qualify as body-building exercise, at least when compared with galloping across the plain or flitting through the air. So fish muscles are not as developed as those of other animals.
Elephants, for example, have to work so hard just to move against gravity that their highly-developed muscles are extremely tough and, as you undoubtedly know, must be simmered for a long time before they are tender.
But more important is the fact that fish have a fundamentally different kind of muscle tissue from most land animals. To dart away from their enemies, fish need quick, high-powered bursts of speed, as opposed to the long-haul endurance that most other animals must possess for running.
So fish muscles are made predominantly of what are known as fast-contracting fibers. (Muscles are generally made up of bundles of fibers.) These are shorter and thinner than the big, slow-contracting fibers of most land-animal muscles, and are therefore easier to tear apart, such as by chewing, or to break down chemically, such as by the heat of cooking. Fish is even tender enough to eat raw, as in sashimi, but steak tartare has to be ground to render it vulnerable to our omnivorous molars.
Another big reason that fish are more tender than other animals is that they operate in an essentially weightless environment. They therefore have little need for connective tissue, the cartilage, tendons, ligaments, and such that other creatures need for supporting their various body parts and for fastening them to the skeletal tree. So fish are practically pure muscle, with none of those tough materials that have to be cooked into submission.
For these reasons, fish flesh is so tender that the main problem is to keep from cooking it too much. It should be cooked only until the protein becomes coagulated and opaque, pretty much the same as what happens to the protein in the white of an egg. They’ll both get tough and dry if you cook them too long.
And why is fish flesh white? Fish don’t have very much blood, to be sure, and the small amount they do have is largely concentrated in the gills. But by the time any animal food gets to our tables, almost all of the blood is gone anyway. The answer has to do (again) with the different type of muscular activity in fish. Because their fast-contracting muscle tissue operates only in short bursts, it doesn’t need to store up oxygen for endurance activities.
Other animals’ slow-contracting muscles must store up endurance oxygen, and they do it in the form of myoglobin, a red compound that turns brown when exposed to air or heat. It’s the myoglobin, not the blood, that makes red meat red.