Ozone generators produce o3 (three oxygen atoms) but it is unstable and rapidly converts back to o2 (oxygen) That extra third atom that brakes loose is very distructive to what it comes in contact with. In aquarium water treatment the ozone is injected into a chamber or a reactor containing the aquarium water. THe exposure of the ozone to the aquarium water kills everything it comes in contact with including viruses. It also oxydizes organics in the aquarium water (kind of like how bleach does). It brakes down waste products to lessen the load on your biological filter. As unstable as it is, it will fairly quickly degass it self out of your water and exhaust it self due to the organics it comes in contact with. It's a decent idea to run the water comming out of your reactor thru activated carbon to remove residule ozone.
It is typically un safe to run an ozone generator with out a devise that monitors the amount of ozone being put into your aquarium water. With a large enough generator, you can easily overdose your water and literally kill everything in your aquarium. THe way I do this is I use a controller that monitors the waters ORP level. ORP stands for oxygen reduction potential,(this is a whole 'nother subject). Ozone will increase you waters ORP, I set my controller to the desired ORP value on the aquarium and the monitor turns the ozone gerator on when the ORP drops below the desired value and off when it get to or slightly above the desired value. THe desired value is a contraversial subject some times, I maintain the ORP value at 425 Mlqv which is considerd too high by most people. My beliefe is that value is in the sump, not the actual aquarium. By the time the water is disolved into the aquarium, the ORP is considerably lower. I also use a residual ozone test kit from Salifert to make double sure ozone is not getting into the aquarium.
Another important issue to adress is the ozone gass being vented out of your reactor. The gas is pretty caustic and can harm most everything it comes in contact with, including you. YOu must vent it out of your room or use activated carbon to filter it out of the vent from your reactor.
I know it sounds like alot of fuss and even perhaps a little dangerous but man does it produce some awesome water quality and almost eliminates all parasites, bacterias & viruses. It isn't cheap to do though, a good controller is a couple hundred bucks and a small ozone generator is about the same. I have friends that have bought used ozone generators from non working hot tubs for pretty cheap and bought a decent controller to run them. They are worth it but do it right or you'll be sorry!!!!!
Joel