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Water Change killed fish

I recently did a water change on my plant tank and all my fish died within a couple of days. They became discolored and exhibited unusual respiration rates and swimming patterns, why did this happen?

I recently did a water change on my plant tank and all my fish died within a couple of days. They became discolored and exhibited unusual respiration rates and swimming patterns, why did this happen?
The following answer assumes that any chlorine or chloramine was removed prior to adding water to the aquarium. If no water conditioner was used, there is a very great possibility that chemicals added by the municipal water supply may have had something to do with these mysterious deaths. Classic spinning movements and darting often suggest chemical poisoning from municipal tap water.

Assuming the water had been conditioned before being added to the tank, this type of situation would generally relate to toxic poisoning of fish. Unusual respiration and abnormal swimming patterns are classic symptoms. First, perform general water tests such as pH, KH, GH, NO2 and NH3/NH4. If ammonia and nitrate are normal in a soft water tank (KH less than 50 mg/L), it is very possible that metal toxicity is a problem, often resulting from a copper pipe change either within the home or a municipal water system repair or adjustment. A quick preventative in this case would be to always run the water for a few minutes prior to using it for the aquarium. By running the water through the pipes, standing water with leeched copper will be flushed and uncontaminated water will be used instead. This is an important trick for new homes with copper pipe as the basic plumbing.

The use of Aqua Plus or Chlor X Change should prevent this. However, with very soft water, high copper leaching, and copper sensitive fish (ex: scaleless species), this situation is possible. In a situation where this is known before the water change, it is recommended to filter the water change water with carbon for 24 hours and then add a double dose of Aqua Plus. In addition, it would be recommended to increase the KH to between 50 to 90 mg/L (3 to 5 dKH), especially for a planted aquarium. In reef aquariums containing corals and invertebrates, even trace amounts of copper can be a problem.

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