Tinkster, First, I'm sorry you've lost fish. Second, sorry for the rant...
I have become very opinionated on this topic, and suspicious of many pond/koi practices. I truly believe that all animal immune systems function best when allowed to mature and adapt to local conditions. When any animal (including human) is brought up in "sterile" conditions, their immune system is completely lost. In other words, we animals need dirty conditions in order to develop our immunity.
I have purchased many Koi that had zero immunity. It's a long haul.
What that means is that only dangerous things can prop up a fishes immunity. Expose a young fish (koi, goldfish, etc) to all the natural pathogenic bacteria that come from decaying plants, fish, parasites, etc. And their bodies will learn how to fend off those pathogens when fed a natural diet. Keep a fish in "sterile", superfiltered, unplanted, clean water, with unnatural diet? They get sick. EVERY time.
Same as humans. Our obsession with a clean house, clean toilet, clean food, clean countertop, antibacterial cleaners,... has led to an epidemic of allergies, immune disorders, autoimmune disorders, staph infections, etc.
In any natural ecosystem, the good bacteria compete with the bad bacteria, and immune systems react and develop. In systems comandeered by people, the good bacteria and bad bacteria are obliterated... leaving only the superbug bad stuff.
Then again, you might just be overstocked.
![Undecided :-\](https://www.worldwidewatergardeners.org/forum/Smileys/smilies_smf/undecided.gif)
If you can't catch them for an accurate diagnosis, then definitely go ahead with the clhorinated water. Not too much at a time. Aerate the water well.