The water companies' remit is to provide safe drinking water for humans, and the problem for fish keepers is that the treatments used to purify tap water are suitable for humans but not necessarily for fish. Additional chemicals are also picked up from the metal supply pipes, which causes more problems for the Koi keeper.

There are known fish toxicity levels for various chemicals, but persistent low levels of toxins can be just as dangerous as spontaneous high ones. Low levels of toxins take a long time to become apparent and are much more difficult and time consuming to research.