I have always thought that aquarium screensavers make the best type of screensavers. This one is nicest free aquarium screensaver that I have ever seen. It is really two screensavers built in to one, first being a Tropical Aquarium and the second is a Marine Aquarium, once you have all of the updates.
With all of the updates you get three additional aquariums and ten additional fishes. Out of the twelve fish that you can choose from you can have up to twenty at any point active in the screensaver. Each of the four aquariums has an aerator built in and you have an option to feed the fish and tap the glass on the aquarium.
The settings allow for quite a bit of customization besides the fish and aquariums, you can change the fog density, the bubbles density and the lighting in the aquarium. In the settings you can choose to show the date and time. There is also a system performance area where you can set the processor usage, the maximum frames per second along some other tweaks.
Another really nice feature is the ability to auto-detect the optimal settings for the screensaver and it seems to work well, but you must already have the screensaver running to have it self optimize.