Here is one of the reasons why I started this poll. I recently saw the video from Tidal Gardens about how to QT. First prazipro and then in-tank formaldehyde. It was all “very gentle and very simple”. I just had a batch of fish to QT and I followed their recommendations, which are not too far off from what experts in the field would say. Except that formaldehyde may be less effective compared to copper. When the rounds of formaldehyde were done, two days later, 3/5 fish had a crazy flare-up of ick/velvet, and the next day they’re dead. I felt terrible about it. I am a chemistry engineer with a PhD degree in life sciences, I used to work at the Sea Life Centre in Belgium working with marine animals and I still screw this up :-(.