I have to jump into this discussion. I am entering my 15th year of saltwater. I got back into this hobby 5 years ago after a twenty year hiatus. I used a diatom filter on my reef tank in the early `90s. Back then I had better than average results keeping multiple fish over five years and breeding Clown fish in 1992. When I got back in, I made sure I purchased a DF. I am a full believer in boosting a fishes immunity through its diet, although I disagree with Paul B on the details. All dry fish food contains fish meal with all nutrients removed for sale to humans and farm animals, or the waste and dead fish from fish farming and terrestrial plants which marine fish have never eaten in the wild. This can be done because of the lack of regulations in the pet food industry. Three years ago was the last time I lost a fish to disease. Since then using a combination of fresh food, UV, and a diatom filter, my fish have been disease free. I do QT, but not to the level advocated by most. Two weeks of eating fresh and live foods in an observation tank which is kept running 24/7 with fish in it. I have not yet had to use medication of any kind. If ich appears, I start the diatom and run it through a 25W Aqua Ultraviolet sterilizer. I have a 50G reef, 120G reef, 120G FOWLR, 29G Seahorse tank, and the 20G QT. There are 34 fish in the tanks including the Seahorses. I did lose some fish in Oct due to a power failure. This means in the last three years, I have purchased over 40 fish in 5 tanks with zero loss of disease. A couple of months ago I had to remove two fish from my 120G reef and into the FOWLR because they were getting too big. Using a home made fish trap, I was able to do this without too much trouble. However, my Purple Tang got all worked up like Tangs do and within a week had a few spots on him. Out came the diatom and UV and within 3 weeks they were gone. Please do not say it is luck. Good water quality, husbandry, and diet can go a long way.