Yep, crazy isn't it? I've discussed this at length with people on an aquaponics forum and their best guess matches yours - that there is leftover decaying organic material in my grow media that's producing (or produced) hydrogen sulfide. The grow beds and hydroton sat all winter and I didn't do much to clean out old roots, leaves, etc.
I did a test where I took out a few pieces of the hydroton (expanded clay balls used as growing media) and left them in a water bottle. The pH of the water in the bottle dropped over the next few days. So it's something in the hydroton. Hydroton is supposed to be pH neutral but obviously mine isn't, or has somehow been permeated with something acidic.
Rather than rip out all of my plants, clean out and rinse the beds and grow media, and start over I decided to just stick it out and keep buffering pH until I neutralize all of that acid. It's been over a month with little progress, so I'm starting to second guess that decision. But I'm too far in now to change course and still have any growing season left, so I'll stick it out for this year and clean it out over the winter.
I'll check hardness the next chance I get. I'm guessing that the system water hardness will be quite high since I've been buffering the pH up every other day with Calcium Carbonate.
And I'll definitely pick up some pelletized limestone. Thanks!
