I'd replace the bulb and plug it in. If the light comes on and water flows thru it (at a slow rate, BTW
), it works. Somewhere, you should be able able to find the recommended flow rate for any brand 25w UV
sterilizer and get the right pump for it. Water flowing too fast thru these things makes'm not work. Target organisms just get stunned rather than burned and killed.
I've heard folks say how clear their water became when they used'm. Some don't use'm at all and say they're not good to use - killing good stuff too. Some claim parasite killing as the reason to use'm. There are a bunch of opinions, but if you've been doing fine so far w/out one, maybe leave it alone.