For algae control it is all about processing as much water as you can rapidly. Oversize the UV and run all of your return flow through it. Algae replicates rapidly and you need to zap it before it can reproduce.
What is your current flow rate of your return pump? If it is not fast enough the UV won't be able to keep up with the algae. This is where the closed loop from DT back to DT is best.
You also don't need to run it 24/7 for algae either. I think
@billyocean uses his sparingly to clean up if he has a bloom.
Lastly you will see some additional heat and water temp increase using a UV. Just something to be prepared for.