Planted Tank Enthusiast
Join Date: Aug 2009
Location: Vancouver, WA, USA
This has happened to me twice. Our water company (and most, from what I understand) does an annual chlorine/chloramine flush, which increases the chlorine/chloramine content beyond lethal levels for some fish. This was confirmed by my tests, which for a week registered chlorine WAY off the scale. This results in fish gasping at the surface, listlessness, and ultimately death. I lost 4 angels last year, and this year about 12 congo tetras. My recommendation, and the only thing that has worked for me, is to age the water at least 24 hours with some agitation (airstone, or powerhead works), and treat with Prime/Amquel Plus, or other chlorine/chloramine neutralizers. I have discus now, and I am SO lucky that I didn't have my discus in the 90 when it killed my congos. My aged water was fine within 24 hours, but that was too late for the fish. Oddly enough all of my catfish survived both incidents - everything from my BN plec and farlowella acus to my otos and corys.
The water company doesn't need to inform the public when they do this, because they only add the chemicals to the legal limits. I'm sure they would tell you if you asked, but there is no obligation to inform you. The only safe option is prevention by habit.