I’ve been using UV lights for 5 years and am quite convinced they can improve water quality as well as water clarity…
I run mine at:
10 gph per 1W of UV…
1W of UV per 10 gal of tank volume…
This is to kill parasites as well as improve water clarity.
For aquarium use I feel if you are going to invest in a UV you might as well get the most out of it and therefore I suggest using the above rates…
For use on an outdoor pond to remove green water use the same suggested rates only change the 1W to ¼ W