Just days after the catastrophe in my tank we left for a trip and didn't return until December 5th. Upon our return, my empty tank that only had the pleco in it,had an enormous algae bloom. This was a clear sign that the tank was lacking bacteria and needed to recycle. This supports your suggestion that during the switch over, which took two-three hours, the bacteria died. The low oxygen in the water from the aging tank only added to the problem since I removed the air bubbler and had no spray bar. The idea that a toxic reaction occurred in the canister from the depleted and stagnant water all makes sense. After I added bacteria to the tank, and some fish, everything cleared up and the tetras have been doing fine for about 4 weeks now. I am in the process of cycling a quarantine tank for new discus arrivals, hopefully in a few weeks. So, I am thankful for your input, it helped me eliminate all the other thoughts, especially when I came home from our trip to see the tank experiencing the algae bloom. I never thought for one minute that the bacteria would die in my canister and then cause so many problems. Thank you for helping me figure this out. I will post a new thread when I get the quarantine tank cycled and expect the newbies.