Source: https://www.pub.gov.sg/watersupply/watertreatment

I know some of us swear by the traditional method of aging water, but the science doesn't lie.

Some time back in the early 2000s, PUB started introducing chloramines into our tap water (ammonia + chlorine). The simple reason was that chloramine is more stable than chlorine and will remain in our tap water long enough to maintain the water quality (kill bacteria). PUB Singapore Drinking Water Quality document states that Singapore's tap water contains 1.9 - 2.7 ppm (average 2.37 ppm) of monochlorine. This is a pretty high amount and will definitely harm fishes.

So how long does it take for chloramines to decompose to safe levels?

A study did an experiment on Fairfax County Water Authority Corbalis Plant water (pretty similar in terms of chlorine levels) showed that it took 155+/- 42 hours for half of the chloramines to decay.

This means that in Singapore's tap water of 2.37 ppm of chloramines, it takes around 6.4 to 8.2 days just for the chloramine levels to drop to HALF (1.19 ppm) which is still extremely toxic to fishes. Now imagine the amount of time it will take to 'age' water till it is safe.

The exact numbers are not important here, what's important is that we understand that 'aging' water is an ancient and unreliable method of water treatment. Using anti-chloramines from reputable brands is essential in keeping our fish healthy.

But but.. I've been aging my water and my fishes are alive and breeding!

Even though most of us know how toxic chlorine/chloramine is to fishes, some continue to insist that aging water doesn't hurt their fishes because "I have been doing it for years and my fish are still alive and breeding".

Let's use cigarette smoking as an analogy to explain how chloramines hurt fish in the long run. Similar to how cigarette smoke is inhaled into our lungs, fish 'breathe' in chloramines (fish don't drink water, they get moisture through the food they eat).

There is no dispute as to how harmful cigarette smoke is to our lungs and the damage it does over time. Smokers can still have children and live relatively normal lives for the most part, but are more susceptible to lung related complications like lung cancer in the long run which will reduce their lifespan. 

Similarly, chloramines essentially burn the gills of fish and reduces its ability to exchange gases. Over a long period of time, microscopic damage to their gills will result in health issues. This will not be seen until it is too late.

Anti-chloramines are cheap, so why skimp on the cost and risk it?