IMO, UV in a home system is a waste of money and potentially dangerous.
If you are running it constantly, you reduce the fishes immune system strength, to a point where they pick-up infections more easily if stressed or the UV is removed completely for what ever reason (hardware failure, warn out lamp, disconnected from the power, lamp crystal is dirty e.t.c). Remember, UV does not kill all parasites, it just reduced their numbers, fish can still get infected in a system running UV, as there are still some parasited presant, but not usually enough to maintain a strong immunity to the parasites
If you are not running UV, you wasted the cash on the unit, that could have better been spent else where.
If you are finding you constantly need to run UV to keep your fish health, something about your care is not good enough for the fish that keeps getting ill, or something in said fishes environment is stressing them. In this case, it would be better to work out what is causing the problem, rather than running the UV and keeping a stressed fish in your care
UV has it's uses, in QT is one example, but only for the first week or two, and in shop systems to name the main ones, but for home keepers, you would really be focusing on keeping healthy fish healthy through good care
HTH
Rabbut