I like u.v. sterilizers but there are some in's & out's about them that play a huge roll in if it will be a signifigant value to you or not. The wattage of the sterilizer and the flow rate are very important. If you have a small sterilizer on a large aquarium, it won't be very affective. Like wise, if you pump water thru your sterilizer to quickly it won't be very effective. The "bugs" in your water that you want to kill need to be exposed to the u/v light long enough to kill them. A good quality u/v should have detailed instructions on flow rates. Also, as was mentioned by another person. You must keep the u/v sterilizer clean. THe film that can build up on the inside will block out the light and decrease effectiveness. They are very usefull at killing free floating algae. It's a common practise on out door ponds to use them to clear up green water. It does not matter if it's fresh water or salt, it does not matter if it's a multi tank system or just one tank. They perform the same function on both. THey are not effective on viruses and their affectiveness is limited at killing bacterias. The u/v light is only good for about 1 year and will need to be replaced annually.
Joel