U.V. can only kill things that pass thru the unit, It also is very dependent on flow rate, watts, and the organism passing thru. Think of it like this. If you go out in the sun you get uv, you dont die unless you spend 5 or 6 days all day under the fierce tropical sun. Now bacteria have no protection from uv, the die almost instantly under the sun, algae are easy to kill too. Parisites have a exoskelton, that absorbs and blocks some of the light. If you slow the flow rate down, it increases the level of uv absorbed and kills them too. pods are larger, and stay as best they can in the tank on the live rock. No harm will come unless they pass thru the uv. I hope that helps you. Steve