UC Roots sterilizes so I’d expect that any organics in the water are probably being killed off by it to some extent, including the hydroguard. At that level it might just be cancelling out the hydroguard with a worst-case that the dead hydroguard becomes food for bad bacteria to consume and grow with.
From what I understand it’s used instead of organic treatments like hydroguard or in stronger concentration (4x the weekly dose) to clean the reservoir every so often, such as every 3-4 weeks sterilize everything in order to introduce new beneficials (which would otherwise die if exposed to harmful bacteria too early). I’ve also read elsewhere that it takes a few weeks for (the bacteria in) hydroguard to establish a mature colony in the root zone, so it’s helpful to defer the first water change for a few weeks to allow for that. If that understanding is correct, then it’d seem counter-productive to use it along with hydroguard.
Unless the water is cooler I would think that the (remaining) UC roots (that didn’t get used up on the hydroguard) will likely gas off and be gone within a few days as it’s basically just watered down chlorine; they recommend to add it again mid week if temps are over 70F.
I experimented with liquid and powder chlorine at first but now only use it for cleaning out the res; 1/2 cup of bleach in water instead of 50/50 vinegar. If the chlorine level (HOCl actually) wasn’t maintained at a constant minimum level then the bad bacteria would take over as soon as it was gone, eventually leading to foul smell when there’s an accumulation of more bad stuff than (that level of) the chlorine can handle.