UV sterilisers only kill water-born things so they don't kill the kind of algae that grows on things but they do kill the lagae that makes 'green water'.
They do kill water-born pathogens but only if they get swept past the UV light before they get to a fish. So for example, an extreme case - a cory sitting next to a whitespot cyst just as it bursts will be infected with whitespot before the parasites reach the UV.
A lot of fishkeepers do not approve of UV filters in the home, or at least they disapprove of running them full time. The reason being that it is mainly stressed fish that get sick and the fish in our tanks should not be stressed. If they are, we are doing something wrong.
The other problem with running it full time is that because the pathogens are killed off, the fish don't develop immunity. Then something goes wrong with the UV and you don't realise, you buy new fish which bring a pathogen into the tank and the existing fish, having no immunity, drop like flies. Something going wrong with the UV could be something as simple as not replacing the bulb before it wears out.
So yes, use a UV, but not full time, and don't expect miracles
As for recommendations, I haven't used one so I'll leave that to someone else.
One thing I will warn about - make absolutely certain that the one you choose is made for aquariums. I read about a month ago on another forum of someone who bought a UV and it killed their fish. When they posted a link to what they bought, the item description on the on-line shop said for use in fish tanks but the packaging said for sterilising water storage tanks. There was no sleeve round the bulb and the fish were exposed to the UV light
