I'm a doctor, not a marine biologist, but with humans, it works like this. It is the humans immune system that fights off disease, not the sterility of the air. A human body becomes infected when it is exposed to an antigen and the immune system is weak due to stress or illness or the bosy has never been exposed to the antigen before and therefor has no antibodies to the antigen. In the fishes' case, for instance, it isn't benificial bacteria in the water that protects it from disease, but the fishes' own immune system. If, for some reason, the immune system is compromised and there is an organism in the water (ie. ick), then the fish will become infected. The bad thing is, is that we never know when the fish may be compromised. I know that ick matures in the sand or gravel, but it eventually has to surface in order to infect the compromised fish. And this is where the UV steralizer comes into play. One thing that it does kill floating around in the water is algae spores. This is benificial in the sence that it cuts way down on algae growth, but does limit the amount available for certain inverts. If you're willing to add a supplement (like DT's phytoplankton), then this is a very good trade-off. As always, just my opinion (except for the human attack by antiigens, that is fact), do what you want, but maybe this will help you.