Things have taken a serious turn for the worse. When I got home on Wed. my taiwon reef was dead. I immediately did a 40% water change and removed all rockwork. Tested water it read 0ppm ammonia,nitrite and nitrate at 2pm. At 6pm fish were breathing heavy so I did another water change 25% (I use chloramx to remove chloramine), No sick fish medication arrived and I began treatment. At 8am on thursday fish were again breathing heavy,did another 25% water change and treated with No sick fish. Came home after helping a friend do some moving etc. at 6 pm and fish were again breathing heavy, so I did another water change. Went back after putting the kids to bed at 9pm and all fish were either dead or in very serious distress. I managed to save only my large yellow labs ,4 msobo males,lateristriga,venustus,and believe it or not the kiwinge in the picture. I moved them into my 20 gallon tank and they immediately came to life. I then removed 100% of the aquarium water from the 230 gallon and refilled the tank. This morning I tested water again and all readings for ammonia ; nitrite/nitrate were 0 ppm. Water looked much better so I added a chocolate cichlid from my 125 gallon. The canary was dead within hours but the BN pleco is still alive. I use the same choramine treatment in all my other tanks and I even had to move my 55gallon twice in 3 days which required 100% water removal and that tank is unaffected. I also treated the 125 with the No sick fish just to be sure I hadn't transferred anything and it is fine. The now toxic 230 sits empty with some kind of toxin in it. I am very careful about cleaning the glass and never spray the top third of the tank when cleaning . I think I can rule out the city water supply because all other tanks are fine. Can a bacterial infection kill fish that fast? Not likely. Something toxic got in the water of that tank alone. I thought, maybe , the bacteria causing the ulcers finally did them in until I added the chocolate after a 100% water change and he also died. I guess I'll go clean out the filters and start over. This is the worst day of my fishkeeping life. Not sure what can be taken from it.I guess next time Ill be more prepared to treat ulcers though the poisoning in the 230 will likely remain an enigma.