I have had tanks for years. I use the same clean bucket I always use to change water. I always thourghly wash the bucket out with scalding hot faucet water. I use the bucket for nothing else. I added the water treatment before the water was added. 2/3 in the first bucket. The only thing I did do wrong is not add a full dose to the second bucket but I did add some to that bucket. Then 1/3 total dose in the last bucket. The total dose was more than called for.
From what I have read, chlorine kills fish over a short period of time, not instantly. If some of the chlorine wasn't instantly neutralized it wasn't much. It would not have killed the zebra within 2 mins of me finishing the water change.
Most plastic will leach chemicals if they are exposed to heat. This includes buckets and hot water. Fish buckets should preferably be made from food grade plastic and only washed in cold water.
As plastics get older, they are more likely to leach things into the water.
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If you have ammonia in your tap water, then you have chloramine in your tap water. This is harder to deal with than just chlorine.
Chloramine is a combination of chlorine and ammonia. They bind together and continue to poison and kill stuff for months. It is used by countries with hot climates or where water has to travel long distances because of this feature (it stays poisonous for months and kills things in the water for that time).
If the water company has done work on the pipes, they will increase the chlorine/ chloramine levels to make sure nothing is alive in the water. They don't normally tell you when they are doing work on the pipes and it can happen at any time of the day or night or any time of the year.
eg: There was a burst pipe in my neighbourhood the other day. I didn't know about it until I turned the tap on and brown water came out. I didn't use the tap for a couple of hours after that and when I did use it, the tap water stank of chlorine. We have chlorine not chloramine.
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Chlorine and chloramine will kill fish within seconds to minutes of a poisoning occurring. It depends on how much chlorine/ chloramine the fish is exposed to. If the levels were higher than normal due to work being done on the pipes, or someone made a mistake at the pumping station and they added too much chlorine or ammonia, then the fish can easily be poisoned.
I experienced this first hand many years ago. I used to gravel clean and drain my tanks, then refill them with the hose. I added a double dose of dechlorinator before filling. I did this for years and one day I did it and wiped out an entire tank of fish within a minute of adding the new water. The water company had done work on the pipes and triple dosed with chlorine and I killed every fish in the tank within a minute of adding the new water.
This is why I always state, all new water should be free of chlorine/ chloramine before it is added to an aquarium. You
never know when the water company is going to add something extra to the water. So it's better to be safe and spend a few minutes dechlorinating and aerating any new tap water before it gets put in a tank with livestock..
In addition to chlorine/ chloramine killing fish, excess ammonia in the water, caused by the water company making chloramine, will poison fish, especially if the pH of the water is above 7.0, which it normally is because the water companies like the water to have a high pH so the chlorine/ chloramine last longer and so the pipes don't corrode as quickly.