Ok, so what is the correct installation, and what products are we talking about, can we get additional details, maybe some images.
Paul whats your UV setup?
You can read the details here:
Aquarium & Pond UV Sterilization | How to use a UVC Sterilizer
The short version is that you need enough UV power transferred to the bacteria/virus to damage its DNA. This prevents it from reproducing and thus eventually it dies off. The amount of power needed to do this varies by what you are trying to kill. Algae is large and can be killed with even very low doses. Bacteria and some viruses are a little harder so they need to be in the UV light for longer in order to be damaged. Some viruses are very tough and can only be killed by very high doses of UV.
The dose is determined mainly be 2 things, the amount of UV light and how long it is in that light. In practical terms this means the UV bulb power and the flow rate of your water. At about a 3rd of the way down the link there is a chart that explains what amount of power and flow you need for each amount of "killing power"
For just algae you can get away with anything up to 50 gallons per hour per watt of UV light. That means for a 10W UV sterilizer you could run it through a 500 gph pump (after head loss) and still kill algae.
For bacteria and easy to kill viruses you can only do a maximum of 30 gph per watt of UV, and you need a turnover rate of at least once per hour. So for a 100g tank you would need the 10W UV hooked up to a pump no greater than 300 gph, but at least 100 gph.
There are other factors to consider as well. This extreme UV light only penetrates effectively through about 3cm of water. This is because high energy light absorbs into the water quickly and then can't be used to fry bad things. If the UV light is further than about 3cm from the water then it isn't doing anything to that water.
There are other important factors as well like dwell time that can modify the above, but in general people run low power UV sterilizers through high flow pumps and then wonder why they aren't getting the results they want.