Sterilize roots how?depends on if your additves caused the foam. if not then its bad. sterilize roots and system and start new buckets.
I got foam slightly around the top layer of the water, get some hydrogen peroxide(H202) 3% from your local drug store, take 10ml per gallon of water and fill up your bucket to where you would without you plant in it. Take your plant and get a big container or another bucket and fill it up with the same ratio of h202/water and soak your plants roots in it for a few minutes. Rinse and wash out the bucket and try again with just plain water. Im not familiar with humboldt's nutrients but if they are organic you will have problems like this all grow. I got aqua shield and have been loving it at 10-15ml/gallon of water and my plants roots exploded! Then they turned male so i killed them. Hope this helps you out man!
You can use the good stuff too but h202 kills living organisms... As in all bacteria good and bad. I use the 3% because I found a site that used that and it was easily accessible. You should be able to use aqua shield with you nutes. After you clean your system and roots fill your system with ph'd water only and some h202 at 5-15 ml/ gallon for a day or 2 and if all goes well and no more foam then we had the same problem. The bad news is it never really goes away but with beneficial bacteria like aqua shield and good res temps i could deal with it and my plant's roots where just huge since I used it. What are your res temps usually?
How do you control where your air pumps pull air from? That's a great idea!