5.5. Chlorine and Chloramine

Roughly 98% of all municipal water supplies in the US have chlorine or chloramine added to the water. This chlorine is added to kill bacteria in the water. If this wasn’t done, people would get waterborne diseases.

This chlorine and chloramine are poisonous to fish in even very small quantities (> 0.05 ppm). It is important to use something called a “conditioner” to neutralize this chlorine and chloramine.

Chlorine Removers or “Conditioners”

My recommendation with conditioners is simple:

Buy ONLY Conditioners that say “Sodium Thiosulfate” on the Bottle in the Ingredients List. Then use 5X the Recommended Dosage.

If the tap water has chlorine or chloramine the water can typically be treated with a conditioner in the water change bucket. Then it can be added to the aquarium. Alternatively, the conditioner can be added to the tank before the change water is added. Dosage should be based on the amount of change water. The amount should NOT be calculated based on the tank size.

When a water system is on chloramine. the chlorine pulses need to be frequent. Depending on the water quality it might be as often as weekly. This is why some suppliers of conditioners recommend five times the level of conditioner for chloramines that is used for chlorine.

Note that for discussion purposes chlorine and chloramine are identical. All conditioners which neutralize chlorine also neutralize chloramine with equal efficiency. There is just some subtleties about treating chloramines (having to do with the ammonia in the chloramine) which require a separate discussion.

Chlorine Pulse Events

When one does a water change with chlorinated water and conditioner, there is a serious problem which can kill all the fish in an aquarium very quickly, anywhere from seconds to days. “Normal” chlorination levels are 0.5 to 2 parts per million (ppm) of chlorine. Most chlorine “neutralizers” and treatments neutralize between 1.0 to 2 ppm of chlorine.

Water suppliers typically will treat with up to 4 ppm of free chlorine periodically. This is what is often called a “chlorine pulse” or a “chlorine bullet”. The 4 ppm of free chlorine or chloramine are the maximum allowed under EPA rules.

The majority of the water supplies in the USA are currently treated with chloramine, which is chlorine combined with ammonia. The nitrogen in the ammonia feeds harmless types of bacteria which create a “biofilm” on the surfaces of the water pipes. When this biofilm gets to a certain thickness it will allow “bad” pathogenic bacteria to reproduce under the biofilm.

So the lines are treated with 3 to 4 ppm of chlorine to remove the biofilms. With new regulations this can be done as often as once a week. Sometimes this is accompanied by what is called “line flushing”, where fire hydrants are opened up. The same level of chlorine can be used when the water has bad tastes due to algae in the water supply.

Note a similar problem can occur when one has well water. Well water typically does not have chlorine in it but it can be very low on oxygen and/or very high in carbon dioxide. These two will do massive fish kills just like chlorine pulses.

Because deaths from “chlorine pulses”, low oxygen and carbon dioxide are very commonly misdiagnosed, it is the source of many fish keeping myths.

These are ALL only myths and are simply not true

We go into each of these myths in more detail below.

There are several things which can kill lots of fish overnight. A heater stuck on, a power outage which stops the aeration in a heavily stocked aquarium, or even a bacterial infection can do it. But if there was a water change just prior to the deaths, a chlorine pulse, low oxygen or high carbon dioxide will be the killer 99% of the time.

If one has fish in stress because of a chlorine pulse understand the excess chlorine is neutralized in the filter of most tanks in minutes to a few hours. So it is of little use to try and save the fish by adding more conditioner hours after the water change.

The best treatment for fish damaged by chlorine is to take the temperature down to 70°F (21°C) and aerate the hell out of the water with a big air pump and airstones. This takes the oxygen level of the water up and gives the damaged gills the best chance of doing their jobs.

Reasons for Chlorine Pulses

These chlorine pulse events are common killers of fish, especially in the summer months. It is sometimes very difficult to closely control the amount of chlorine added in these events. While the EPA limit is 4 ppm the author measured 11 ppm chlorine in Tempe Arizona once.

Many hobbyists on social media report fish dying after a water change even though they used conditioner. Invariably this turns out to be chlorine poisoning. Mike Mass lost 180 angels and cardinals overnight (ouch). Adam C. lost several aquariums. Amy Gage lost a whole tank of fish. Mike Randall lost 70 Africans in 3 days. Colby Ladd lost 32 Lake Malawi’s. The author lost about fifty mbuna to a chlorine pulse.

Unfortunately these mass deaths are often blamed on a host of other reasons: ammonia poisoning (only very slowly kills), pH shock (a myth), temperature shock (another myth), electrical shock (still another myth), heavy metals in the water (laughable!), etc. ad infinitum.

Symptoms of Excess Chlorine

This excess chlorine will often give very random deaths. Some fish will keel over and die immediately, others will be just fine. It tends to affect one species more than another. It all has to do with the thickness of the mucous covering on the gills. A thick mucous covering can prevent chlorine from killing the cells in the gills. Some species just have thicker mucous. And some fish can live on reduced oxygen from impaired gills while others succumb to any reduction in body oxygen.

Here are three fish which were hit with excess chlorine:

Here is a pleco which was hit with chlorine and has scaly skin as a result:

“Natural” ways to Remove Chlorine and Chloramine

Note there are many “natural” methods pushed on social media for removing chlorine and chloramine. These “non-chemical” methods include: letting the water sit for 24 hours, aerating the water, spraying the water through shower head and UV light.

I didn’t test these methods of removing chlorine and chloramine because they will ALL be very highly variable depending on the chemistry of the water and the chlorine or chloramine. So I might find that in my water chemistry a 24 hour “outgassing” works well. A reader might then use a 24 hour outgassing with their water and end up killing a tank of fish. And I do NOT want to be responsible for killing a tank of fish.

The various sulfite reducing agents are a cheap, very safe and extremely reliable way to remove chlorine and chloramine. Any other way is not reliable and simply shouldn’t be used.

“I Don’t Use Conditioner!”

Now there are some uninformed (I prefer a different term) who have chlorinated water and don’t treat for chlorine and claim their fish are unaffected.

The last three mechanisms take time to remove the chlorine. So while the hobbyists may see no apparent damage, in a chlorine pulse the gills of the fish may well suffer long term damage. Any hobbyist with chlorinated water should either add a lot of sodium thiosulfate conditioner or test the water for total chlorine and add conditioner accordingly. Not adding conditioner risks long term unseen damage to the fish. Not doing this also risks a mass die off of fish, which is not a pleasant experience.

Fallacies About Excess Chlorine

This is a repeat of a section above where I go into more depth on the subject. If someone comes up on social media and says they did a water change and some or all their fish died overnight there are typically several incorrect responses.

All these things are just common myths in the hobby. The “pH shock”, “temperature shock”, poison rocks, electric shocks, and bad conditioners are all simply false myths parroted over and over again on social media. None of them has EVER killed a fish. And ammonia kills very slowly, over many weeks, unless your water is over 9.0 pH. We go into each myth below:

Mythbuster: pH Shock Doesn’t Exist

Fish in the wild constantly swim though water with wildly different pH’s as the sun, plants and carbon dioxide interact. So all fish can take very large rapid changes in pH. This myth is debunked in the following link:

4.8. Stability is not Important

Mythbuster: Temperature differences of less than 10°F (5°C) are Harmless

As anyone who has swum in a lake on a sunny day can attest, there are very large “thermoclines” with radically different temperatures in natural lakes. Fish constantly swim through these thermoclines with no damage. So all fish can take rapid changes in water temperature. This myth is debunked in this link:

4.8. Stability is Not Important

Mythbuster: There are NO Toxic Rocks

There are no “toxic” rocks. It is very common for hobbyists to do a water change when doing something like changing the décor and adding new rocks at the same time. They then come on social media and say things like: “The rocks from my local fish store poisoned all my shrimp overnight!”. This myth is debunked in the following link:

14.3. Rocks

Mythbuster: Fish DO NOT get Electrocuted

It is virtually impossible to electrocute a fish as few tanks have a ground in them. Here is a link which debunks that myth:

14.11. Electrocution in the Aquarium

Mythbuster: Too Much Conditioner will only Rarely Kill Fish

And most chemical conditioners are based on a very simple chemistry. They have sulfite groups (SO₃) which are oxidized by chlorine to sulfate groups (SO₄). These sulfite reducing agents are all very mild and can’t possibly kill fish at recommended dosages. So a “new conditioner” cannot kill the fish if used at the proper dosages. There are some conditioners which can produce some formaldehyde in the aquarium. But even these conditioners are quite safe at the recommended levels. Here is a link which debunks that myth:

5.5.3. Water Conditioners

Note that there are a few “natural” products which claim to remove chlorine. Avoid these products, they are not water conditioners and do NOT remove chlorine. There are also some forms of “API Stress Coat Aquarium Water Conditioner” which do NOT have a red plus sign (+) on them. These API conditioners do NOT remove chlorine, even though their label says they do.

Mythbuster: Ammonia does not kill rapidly.

When there is a large loss of fish dying overnight reported in a Facebook forum there are immediately many people posting that the fish died because of ammonia poisoning, “obviously”. The posts are simply parroting what they’ve read somewhere else. They are simply wrong. Ammonia kills gradually over time. It takes weeks or months to do its evil. Chlorine kills overnight.

This is typical. A hobbyist believes that because they cleaned their filter they got an ammonia spike and it killed their fish. This is incorrect. They did a water change with water which was super-chlorinated and killed their fish.

Ammonia damages fish by passing through the gills of the fish as the gas ammonia. The ammonia then converts to ammonium in the bloodstream of the fish. This ammonium imbalance then damages the internal organs relatively slowly. If the fish is going to die it will generally take at least several days and more likely weeks, months or even years for that to happen.

For more on how ammonia kills and at what levels it kills go to this link:

5.2. Ammonia Toxicity

Chlorine is 100 to 1000 times more toxic than ammonia. Chlorine kills exposed tissues like the gills of the fish outright. When the cells in the gills of the fish die, the fish is starved of oxygen and may die anywhere from minutes to days after exposure to the chlorine. Therefore, chlorine kills fast and ammonia either doesn’t kill or kills slowly.

In the summer months many municipalities must super-chlorinate. They have no choice. When this happens there will be a flood of Facebook posts on how whole aquariums of fish died overnight. The replies and “advice” always are that ammonia killed the fish. The simple fact is that:

Ammonia does not kill overnight! Chlorine kills overnight!

The Truth: If one asks, the hobbyist with the tanks of dead or dying fish has almost always has done a water change with chlorinated water just prior to the fatalities. It is just very common to have chlorine spikes kill all the fish or most of the fish in a tank. VERY COMMON!

For more information on chlorine and chloramine click on the following:

5.5.1. Chlorine in the Aquarium

5.5.2. Chloramine in the Aquarium

5.5.3. Conditioners

5.5.4. Stress Coat Products