Placebo effect maybe. The cheaper units wont kill parsites unless you slow the water flow to about 1000 L/h
The cheap UVs are good for freshwater or ponds to kill algae, they are no good for saltwater
Here is some info what output is required to kill protozoa. i doubt any cheap Uv will do it.
on the "kill power" needed for the corresponding Micro-Organisms.
Waterborne Algae 15,000 to 30,000 æW sec/cm2
Common Bacteria 15,000 to 30,000 æW sec/cm2
Protozoa (Ich) 45,000 æW sec/cm2
Fungi 45,000 æW sec/cm2