UV units can be useful for some disease control or for algae control. Since a UV never sees every drop of water in the tank, it is not a cure for any disease, but can help in the battle while you are using the proper medications. Similarly, it never removes all algae spores so you will still see some algae. I do not use a UV for the simple reason that I have no need for them. I do not mind some algae in my tanks, I consider it a plant like any other plant. I am very careful about the fish I add to my tanks so I worry very little about disease also. In the last 7 years since I got back into the hobby I have had one small tank with an ich infestation and that is it. A bit of salt and the ich was gone, although it took a week or so to kill all the parasites.
For me, a "magic cure" for most problems is a big water change. If something about a tank of fish looks wrong to me, usually in the form of strange behaviors, I do a huge water change, try changing their diet and almost always see the fish bounce right back to vibrant good health. I seldom know what was wrong but figure if the water quality is kept very high, the fish will heal themselves. It seldom works out to be the wrong answer.
If you are interested in reducing water change frequency, try using natural plants to tie up the metals and nitrogen in your tank water. That means staying away from the planted tank people and using the "poisons" in your tank water as fertilizers, not the added ferts that plant people will encourage. I run a Natural Planted Tank, NPT, that gets a 6 months water change whether it needs it or not. The fish in that tank are abundant and probably overfed while the water in the tank is pristine. As WD said, a water change is used to replenish the trace elements in the water. After about 6 months, I start to worry that my plants may not be getting enough minerals so I do a large water change. Right before the change I measure my nitrates at less than 10 ppm, better than my tap water, and other measurements show zeros for ammonia and nitrites. The water's pH is close to my tap pH so I feel fairly good about doing that huge water change. This is the tank I am talking about. As you can see, the fish population is far from light in that almost unfiltered tank. It does have a sponge that gets cleaned about as often as I do a water change.