Parameter

Chlorine and Chloramine

residual disinfectant carried in from the tap

Reported in ppm as Cl₂

Where a reading sits on this scale

Target
0 ppm
In range
Detectable
0.05 – 0.5 ppm
Act now
Tap-water range
0.5 – 4 ppm
Act now

What it is

Public water systems in the United States are required to keep a disinfectant residual in the distribution network, and the two in common use are free chlorine and chloramine — chlorine combined with ammonia. Both are doing a job you want done on the way to your house and no job at all once the water is in a tank. Chlorine is volatile and will leave standing water on its own given time and agitation. Chloramine is deliberately more stable, which is exactly why many utilities switched to it, and it will not gas off on any timescale useful to you.

Why it matters

Two separate problems. The first is direct: disinfectant damages gill tissue at concentrations well below what you can smell. The second is quieter and catches more people — the same residual kills the nitrifying bacteria living in your filter media, so rinsing a sponge under a running tap can flatten a working biological filter in under a minute and produce an ammonia reading two days later with no obvious cause. Chloramine adds a third wrinkle: when a conditioner breaks it apart, the ammonia half is released into the water, so a freshly treated water change on a chloraminated supply can put a small ammonia reading on your test kit that is entirely expected.

What each band means

Chlorine and Chloramine bands, in ppm as Cl₂
Band	Range	Status	What it means
Target	0 ppm	In range	What any tank should read. Treated water and an established tank both sit here.
Detectable	0.05 – 0.5 ppm	Act now	Something untreated went in, or the dose was short. Treat the water and check that the conditioner is not past its date.
Tap-water range	0.5 – 4 ppm	Act now	Typical of water straight from the faucet. EPA sets the maximum residual disinfectant level for both chlorine and chloramine at 4.0 mg/L as Cl₂ in drinking water — a level that is fine to drink and not fine for gills.

How to test it

Test the tap first, not the tank. Knowing what arrives at your faucet is most of the answer.
Ask your utility which disinfectant they use, or read the annual consumer confidence report they are required to publish. Chlorine and chloramine call for different handling.
If you use a strip with a chlorine pad, read it immediately — those pads change fast and then fade.
Test again after treating and after mixing. A conditioner added to a bucket and not stirred has not reached all of it.

How to move it

Treat every drop of replacement water before it enters the tank, and dose by the manufacturer's label rather than by any number from this site.
Never rinse filter media under a running tap. Rinse it in a bucket of water you have taken out of the tank, and put it straight back.
On a chloraminated supply, do not rely on letting water stand. Chloramine is designed not to leave.
If you use a python-style hose that fills the tank directly, dose the conditioner for the full tank volume into the tank before you start filling, and refill in stages rather than all at once.
Check the date on the bottle. Conditioner is cheap, and an old bottle that has lost strength presents as a mystery ammonia problem.

Where the reading goes wrong

Assuming a chlorine smell means chlorine and no smell means none. Chloramine is much less noticeable and is the one that persists.
Testing tank water for ammonia right after a conditioned water change on a chloraminated supply, then treating the result as a filter failure. The ammonia came from the chloramine molecule.
Treating a bucket, then topping it up with more untreated water.
Assuming a carbon-block drinking filter on the kitchen tap handles it for the whole house. It does not, unless the tank is filled from that tap.

Problems that start here

Act today
My new tank is reading ammonia and there are already fish in it
Act today
Ammonia is at zero but nitrite will not come down
Act this week
The water has gone cloudy and white
Act this week
The filter output has slowed to a trickle

Published sources

Published figures on this page come from the documents above. Where a value depends on species, pH or temperature, that is stated rather than averaged into a single number.