XP !Plus has the very real looking fish screensaver. You get some fish. You have to buy more from another company.
Those other free screensavers probably have adware/spyware included and infect you if you installed them.
After trying out different fish screensavers and such, I have yet to find a marine aquarium screensaver where the fish move like they do IRL. They all claim to have "realistically life-like fish", but the fact is, sometimes the animations for the different fish are just plain silly. When you do 1 type of fish, it can be pretty good, because there is only 1 type of fish to animate, but having only 1 type can get ... monotonous. The best-looking one I have seen, Serene Scene's Marine Aquarium Screensaver, has well-modeled fish, but, they do not move like they should. They all swim around with their tail fin. This is not the way all fish swim. Example:
Parrotfish, Wrasse, Regal Tangs, and various other reef fish swim with their pectoral fins. Their tail fin acts solely as a rudder. Some pectoral swimmers will flick their undersized tail for a short burst of speed.
Most species of Triggerfish, Filefish, and Eels swim with what I call SlugDrive?. They wave their extra long ventral and dorsal fins to provide thrust. Surprisingly, they can move quickly this way, but they can unfold their normally-folded tail fin and flick it for a short burst of speed.
Some species of Triggerfish, Pufferfish, and the impressively huge Mola-Mola (Tropical Sunfish) thrust with their ventral and dorsal fins. This is different from SlugDrive? in the fact that their fins are too short to produce the SlugDrive? effect, so they flap them side to side.
Most large, high speed, and game fish swim with the tail fin, but some reef fish swim with their tail fin as well. Some small reef fish swim with a combination of their tail and pectoral fins, a kind of "all fin drive" that provides a combined thrust.
Anyways, is there anyone that knows of a Marine Aquarium screensaver that has realistically-swimming fish? Is there one in development?