Boiling water for twenty minutes is a pretty good way of getting rid of all the dissolved gases. Sure, the chlorine will go, so will the oxygen, the CO2 for the plants, the nitrogen, you name it.
Waste of time, money and energy
And you’ll have used quite a bit of electricity boiling the water, which you now need to re-oxygenise by pouring the water into a 5 litre plastic bottle three quarters full and giving it a good shake. Doing that a few times would have gone a long way to kicking the chlorine out anyway without losing the gases in the atmosphere from the water, and saved your electricity bill.
A more excellent way
If you are doing water changes nice and regularly and therefore in any given day are only changing something like 10% of the water, then the level of chlorine won’t upset most aquarium fishes too much. I never bother with it myself and the fish always look happy enough after a change. Warsaw Poland has a bit of chlorine but maybe there are places where there is a lot more chlorination going on, I don’t know. But if the water is safe to drink from the tap then a ten percent change is unlikely to upset most fish so maybe you are just making a rod for your own back…”
Strangers: A Memoir of Marriage
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If you avoid those problems, there is nothing intrinsically cruel about the aquarium hobby. Our well-maintained aquarium fishes have a much better quality of life much better than that in the wild. Whole species are now being maintained in hobbyist collections which are extinct in the wild. The German hobby and Hans-Georg Evers in particular brought the Noah’s Ark capability of our hobby to peoples attention already in the 1980s.
