It has been said that the residual Chlorine in municipal tap water is so toxic that concentrations of 0.25ppm can kill 100% of your fish and lower levels can most likely still harm or kill some species. Municipal tap water might be as high as 4ppm in some places. That is very concerning.
I decided to run some experiments to see what is really happening with chlorine and chlorine removal in tap water for use with aquariums.
First I decided to verify weather I had Chlorine or Chloramine. I used a sample with some dechlorinator to remove the chlorine from a water sample. After that I used a Seachem Ammonia Alert to verify there was no ammonia present in any detectable level. I therefore conclude I have chlorine and not chloramine. I also looked it up online to see what the water treatment people say. The maps that shows where chloramines are in use does not show my area using them.
I used a Hanna Checker Total Chlorine Model HI 711 to measure total chlorine concentration for these experiments.
First I measured the chlorine at the tap without any aeration.
Results: 2.1ppm Total Chlorine.
With a standard kitchen sink aerator I repeated the test.
Results: 0.25ppm Total Chlorine.
I tested the ability of a very inexpensive powdered carbon filter out of a 2 pack from a local hardware store. No airator was used before filtration.
Results: 1.5ppm Total Chlorine.
The aerator works much better then this filter.
Next I switched to a high quality chlorine carbon block filter from BulkReefSupply.com.
It is the BRS Chlorine and VOC -5 Micron filter.
BRS Chlorine & VOC Carbon Block Filter - 5 Micron - Bulk Reef Supply
No aerator was used before the filter.
Results: 0.02 ppm
I decided to test an old rumor that has been hanging around the hobby that if you just leave the water in a bucket overnight the chlorine will off gas all by itself.
I left 5 gallons of water pre aerated down to 0.25ppm total chlorine in a bucket for 7 hours.
Results: 0.21ppm Total Chlorine.
Next I tried using a power head. I intentionally put it deep in the bucket so that there was no turbulence in the surface. I wanted to test the idea that you need to break the surface tension to get any gas exchange. I did not break it in this test. Just moved the water around for 2 hours.
Results: 0.24 ppm
Next I tried using an air stone (1" size) and a Model 10 Whisper air pump. This definitely breaks the surface tension but not a lot. The results were a bit strange.
After 15 minutes 0.14ppm Total Chlorine.
After 9 hours 0.12ppm Total Chlorine.
Not entirely sure this test worked right. Perhaps a trace amount of dechlorinator got into the test sample.
For the last test I used a power head below the surface with no surface agitation. I used a cheap generic dechlorinator.
After 10 minutes 0.00ppm Total Chlorine.
The Chlorine was probably gone much faster but it took my that long to get everything set up and the test ready and run.
Conclusion. Using dechlorinator is much faster,safer, and cheaper then trying to use carbon filters or aeration and then having to test to see if they were effective.