Being the OP, I am making a final update to this old thread (experiment is now ended), as promised. Much more happened, as a result of the two-hour accidental UV-C exposure than initially noticed in post #46.
NOTE: I recommend against using these type of unprotected UV-C lamps in our tanks! They are dangerous.
The UV sterilizers that we use in out tanks are generally UV-C, but they are encased for protection of our animal and plant inhabitants - and us. Well, there are also aquarium UV-C sterilizers for sale that have no protection and this is what I used. The intent, if you read through the posts, was to destroy the periphyton without killing anything else. I suspected that, with the right amount of exposure, this can be done. However, my timer failed during this experiment and I discovered that the UV-C had been on for about two hours (I had intended several exposures of 1 minute each). The result was a Chernobyl-type event. All plants within ~one foot radius of the light gradually died down to about an inch above the substrate. Recovery was extremely slow (DNA damage?), taking about two months just to reach a height of several inches and fullness did not return for about three months. They are fully recovered now.
All sensitive fish, such as neons were wiped out (as well as all the CS and Amano’s). On a Kissing Gourami, I could see blistering on it’s body, like a bad sunburn, and it wallowed on the substrate for a week before gradually recovering - a similar pattern with the other survivors.
Initially, my interest was in seeing if I could reduce GSA and some BBA that formed on lower, older, leaves and, occasionally on some apparatus in high flow areas (as BBA is accustomed to doing). PO4 dosing at high levels (5-10) didn’t help the nagging GSA. Note that all algae was very subdued, it was just here and there and very controllable, but I was striving for zero observable algae. Of course, I killed all life with this UV-C, including algae. Following the disaster, and thanks to @Edward
’s recommendation, I added some ramshorns. Without any other changes to my setup and water parameters, no algae ever returned. I don’t believe that the ramshorns eat GSA or BBA, but I am convinced (see below) that, at least ramshorns, are capable of either preventing biofilm development or those biofilm aspects that support algae.
I didn’t give up on the UV-C experiment, though. I set up a 2.5-gal tank I use for QT and recreated the environment of my display tank sufficiently to grow healthy plants and GSA and BBA (as well as other algae varieties). In this case, I do have biomedia in my filter, unlike the display tank. I then placed the UV-C fixture into this tank and ran it for 2 minutes every 4 hours. It did kill the algae and the algae did not return. However, the interesting thing was that it was like moss on the north side of a tree in the northern hemisphere: where the UV-C hit the surface of things, there was no algae. About two months ago, I removed the UV-C and added a lot of ramshorns. The algae did not return to the area burned away by the UV-C and only now is the algae on the dark side gone.
One experiment probably tells us nothing but, to my way of thinking, the coincidences of what the snails seem to be doing in both tanks is enough for me to suggest that this is a good superstition: add snails, at least ramshorns, and biofilm will be controlled/eliminated and this does affect algae.