Well, Chlorine and Chloramine are added into water to kill germs thus making it palatable to us humans.
However, Chlorine & Chloramine has some caustic effects of the gills of fishes which render them useless / degrade after a period of time. Which means that the fishes that are exposed to Chlorine or Chloramine will not be able to breath and thus suffocate. And because the effects are not immediate, our poor fishes will "slowly" suffocate to death!
The only way to prevent this is to nutralize or remove Chlorine & Chloramine. There severals ways:
1. Use Chlorine & Chloramine Remover (Cheapest)
2. Use RO / DI water (Ex)
3. Use Specialized Tap Water Filters (Not so Ex as RO / DI)
Hope this help.