If a UV
unit is hooked up correctly and the flow rate thru the system is correct (slow enough) to provide a proper exposure the UV
filter can effectively sterilize any water flowing thru it.
How will this effect exiting algae that may exist on the substrate or the rocks? Well I have to think it would have little effect as the UV
is not removing the fuel that is driving this algae growth.
units are only effective against parasites or bacteria or algae spores that are free floating in the water and also that get exposed to the UV
lite at the correct exposure rate.
Often to many people put UV
units inline with their filter systems at a flow rate several times higher than is effective.