They are most useful in getting rid of the yellow color that often develops in aquarium water over time. The most common chemical filter is activated carbon, used mostly in canister and power filters, it traps many substances in it's minute pores. Notice that many of the common aquarium medications including antibiotics are removed by activated carbon and therefore should not be used when treating the aquarium for disease. Other filtration - Foam Fractionators, or protein skimmers , work by passing a large stream of small bubbles through a column of aquarium water. One of the drawbacks to skimmers is that they remove trace elements and iodine which, if needed by any of the aquarium inhabitants, must be replaced periodically.
Ultraviolet sterilization works by passing water through ultraviolet light.
If the water is exposed long enough, at a high enough concentration of light, bacteria, algae, and parasites can be destroyed by the light. According do Julian Sprung and J. Charles Delbeek in their book The Reef Aquarium Volume Two, ozone is useful mainly in large public aquariums to keep the water from coloring. They also state that aquariums that use ozone generally maintain a slightly higher nitrate level. The first thing you probably want to know is how many fish you can keep in a particular sized aquarium. The most common formula for the amount of fish you can safely keep are based on the number of gallons of water in the aquarium.