Correct, the UV will not effect the beneficial nutrients of clay, but there are beneficial organisms that live on the clay, which this clay is often used to help jumpstart a biological filter due to the beneficial minerals and microorganisms. Still though, I don't think much of the clay will actually float through the UV unless you are putting it somewhere directly in line to the UV. If there's any concern, then, after the clay dosage, just turn off the UV for a few days is not a big deal then turn it back on.
Interesting read for ya from this forum...
Montmorillonite clay how many members use it and and of its importance to koi and goldfish
Also,
calcium montmorillonite clays is an extremely common mineral additive to homemade fish food paste recipes, which is a big deal with koi. Many scavenger fish, like koi and goldfish, actually look forward to eat mud for part of their mineral intake. The clay also contains a very low attraction polymer so it can help to
slightly clear up the water. One of the most popular formulations, amongst serious hobbyists, I come across reference Ultimate Koi Clay (shown in the aforentioned hyperlink) and
Thrive Koi Clay. I choose whichever I can buy the highest quantity of.
Here are links of only
some of the recipes that might be of interest that involve this clay...
Good Growth and Colour Homemade Food
A look at alternate foods for your koi and goldfish
Updated alternate food for koi and goldfish
Mixing crushed barley pellets in with clay is quite popular and this digested barley releases chemicals from the fish waste that, according to anecdotal experiences, helps with algae control. Whether this is true or not is quite irrelevant, since it is not the primary reason why folk use barley plus clay recipes, so I view it as a nice potential secondary benefit.
Honestly, calcium montmorillonite clay is only one of two products that I use in my pond as part of my "illness prevention" pond program. The other product is
Koi Zyme, which is mainly a personal thing since I am often recommended it from other serious hobbyists that I trust.