Entire species dead after water change

[Toss]

I have been using water conditioner since I got my 75 gal. Before I never did, just because I don't know much about it. I have been in the hobby for 20+ years, both in Canada and in Indonesia (3rd world country). I always use tap water. God knows what is in the water over there, sometimes green...sometimes has tubivex worm .. sometimes the human got sick from it. I don't do large and often WC, never have, but my tanks are always heavily planted. Maybe it acts like a buffer. My fish always seem happy and breeding. The truth is, like Sue said ... it is a peace of mine (like a seat belt)knowing that you did what you can to prevent your fish from dying.

[Demasonian]

Sorry for your loss, especially after the long wait, it has to be frustrating.

My money would be on either Chloramine or a pH shift that your cyps couldn't handle.

Just a thought, and this may be completely out on a limb with no scientific grounding in reality...you mentioned that your bottom dwellers weren't effected but that the cyps were in trouble 20 minutes after the WC. Could it be that because cyps hang out at the top of the water column, that they were exposed to a much higher concentration of chloramine (or even just water with a different pH than the rest of the tank) than the multis below, before your filtration could evenly disperse the "new water"?

Also, do you do your WC's straight from the tap (ie via python) or do you aerate overnight? Do you buffer at all?

About pH changes, I've noticed that standing tap water for me on city water will fall from around 8.9 to 6.8 in a couple of days because of a lack of natural buffering capacity! I buffer with baking soda and epsom salt to keep it around 8.4.