When dechlorinating water you need to keep in mind that chloramine breaks down into
Make sure when you use such products and when your water contains chloramine, that the product you use neutralizes both, or an ammonia spike will occur.
This can bring stress about and may result in parasites appearing on the fish. Ammonia gas, the toxic part, is very soluble and diffuses in the water very quickly. Fish are stressed because the gas affects their gills, making it more difficult for them to absorb oxygen. To obtain more oxygen they need to breath (gill) harder and that causes additional stress.
Too much stress weakens fish very rapidly. The result is that they cannot resist parasites and become infected. Infected fish allow parasites to multiply. This forms tomites on the fish. The tomites fall off, and result in large amounts of free floating parasites looking for a host (=fish) when the tomites release new parasites.
This is usually when other fish get infected, as they too are stressed and cannot resist the attacks of more and more parasites. Eventually they become infected as well.
If your fish are infected, a good way to treat them is by using Vitamin C at elevated doses, making sure that you carefully monitor what the Vitamin C does to your dKH (buffer) and pH. Use Vitamin C that is made for aquariums. Our web site's main page contains a direct link to the Vitamin C free article. It is strongly suggested that you read it.
A. Thiel, Jan. 1996