How It Works
Within a certain wavelength light has long been known to have a germicidal effect in both water and air. Broadly speaking these wavelengths fall between the blue/violet range of the visible spectrum and the shorter invisible wavelengths of X-Rays. Measurements of these wavelengths are made in nanometers (mm) and the most effective wave-length for germicidal purposes is 254nm, although the actual lamp produces ultra-violet light may cover a wider range from 100-280nm. This range is normally referred to as UVC Radiation. If aquarium water is passed close to a lamp producing UVC Radiation, most micro-organisms will be destroyed or severely disrupted. These organisms may include various bacteria, fungal spores, free-swimming algae, viruses and dinoflagellates such as Oodinium and Cryptocaryon species.

To be fully effective several conditions must be met: the water passing around the lamp must be clean and free from suspended particles and discolouration; the flow rate over the lamp should be correctly adjusted; the lamp should be corrected adjusted; the lamp must be of the right wattage for the volume of water it is to treat and the housing must be kept clean and the tube renewed regularly.
Noting these conditions it will become increasingly obvious that the ultraviolet sterilizer would be best be connected to the return hose of a suitable canister filter or such pre-cleaning device. This filter should be packed with filter floss to trap any particulate matter and a good quality activated carbon to absorb any discolouration that may interfere with the treatment of the water. Optimum flow rates through the sterilizer are essential. Too fast and the organisms will not be destroyed; too slow and unwanted organisms may multiply faster than they can be treated.
The correct flow rates for marine tanks are to found on table below. 

UV Specifications for Marine Tanks

Lamp – Aquarium Size – Flow Rate per Hour

8 Watt – up to 40 gal (180 l; 48 US gal) – about 160 gal (725 l; 190 US gal)

15 Watt – up to 80 gal (365 l; 96 US gal) – about 300 gal (1360 l; 360 US gal)

25 Watt – up to 100 gal (455 l; 120 US gal) – about 400 gal (1820 l; 480 US gal)

30 Watt – up to 150 gal (680 l; 180 gal) – about 450 gal (2045 l; 540 US gal)

50 Watt – up to 200 gal (910 l; 240 US gal) – about 500 gal (2275 l; 600 US gal)