Dermp
Here's the real low down.
If you want to run a UV to control parasites, you have to size and operate it correctly. More than likely your LFS is promoting a under sized UV, for no other reason than UV manfs way over-rate their products. They are primarily used as water clarifiers in fresh water tanks and ponds and are rated as such.
You mentioned a 25w UV. Do you know if it's a N(ormal)O(utput) florescent bulb? a H(igh)O(utput) bulb? or one of the newer T-5 bulbs?
Do you know the manfs flow rates for killing parasites?
You will need a flow rate aprox 4-5 times your total system volume. My suggestion is to contact the manf and get their recommendations for flow rates. Then get the next larger size.
As far as killing beneficial organisms. These organisms have a tremendous rate of reproduction and are substrate oriented. A UV has very little effect on them.
As far as wasting your money, that's up to you. Used properly and sized properly, UV's do the job and then some. Anyone that doesn't research and operate one correctly will be wasting their money though.
We are limited here with what we can treat with copper ( can cause severe injuries to organs, including reproduction ), formalin , and other meds. We rely almost entirely on UV in most instances.
The common parasites that you would be trying to treat, you can introduce into your system by adding any hard surface material to it. That includes rock, corals, inverts, along with fish.