I live in Zurich, and got a tour of the water refinery here; they use shrimps for the same purpose. I can recommend the tour as they're also a bunch of insane overachievers who use, essentially, <i>all</i> the ways to purify water, simultaneously. They say that it's unfair to compare Zurich tapwater with bottled mineral water, as the tapwater is substantially better.,(reply>), @RedPuma90 the water quality in northern Germany is comparable poor compared to mountainous areas like Switzerland south Germany, Austria and most of the Balkans ground water no matter how good your treatment is is never even close to spring water that in some cases spent thousands of years outside of the water cycle,(reply>), @RedPuma90 The UK has a lot of Victorian pipework which is still used today, so it leaks like a sieve. The result is that they have to crank the water pressure way up to prevent contaminants leaking in (which of course makes it leak even more), and even then they have to heavily chlorinate. London's especially bad; running a tap there produces noticeable fumes.,(reply>), ​@hjalfi this is the same in the whole of Germany, tap water is only clorinated in very special circumstances, like when they detect a contamination (which is rare) or are changing big parts of the water infrastructure.,(reply>), I remember as a kid that I learned, here in Germany we use some fish for the same reason.,(reply>), At Grenoble (France) i have seen trout.