One more thought. I know I sound like a shill for this product, but what about going to a Dutch Master "Zone" approach, instead of hygrozyme. Zone creates a sterile environment, similar to h2o2, only it lasts longer as it doesn't disapate out like h2o2. It cleans, conditions, and sterilizes the nutrient solution. Hygrozyme, as you know I'm sure, breaks down dead organic material and converts much of that into food for the plant. But, it may not always be friendly with recirculation, and it doesn't really sterilize your system - though it will definitely help aid in sterilization.
When I started growing it was all DWC, with pretty high res temps at times(into the 70's) as long as I put Zone in every one of my res changes I had no problems with roots. In fact, in the grow box I had, I was never able to remove the reservoir to clean it during a grow. All I could really do is drain old nutrients, and refill with new nutrients. Zone was my one way to keep things clean and sterilized, and it did a great job.
In my opinion, it's amazing stuff. Sorry if I've been a broken record recommending this product, but it's been a crucial ingredient in all of my grows so far, and I've seen how it can even take brown roots and brighten them right up - it's a good emergency "fixer" too. I feel like the reservoir just runs cleaner, in general, with it (I have run out of it a couple of times, and noticed a difference right away without it).
Just sharing what works well for me in the F & G - if you ever want to try a different approach.