A rough import, needs some love. Translating has not started yet.

5.5.1. Chlorine

5.5.1. Chlorine in the Aquarium
5.5.1. Chlorine in the Aquarium

Most municipal water supplies in the developed world use chlorine to make the water safe to drink. The author spent several weeks close to death after brushing his teeth with poorly chlorinated water from a municipality in the “undeveloped” world, so he will tell you chlorination is a very good and necessary thing.

But chlorination is not a simple “add chlorine” and walk away proposition. Water treatment plants typically aim for a MINIMUM of 0.5 parts per million (ppm) of chlorine. Chlorine gas kills bacteria by oxidation. The problem with chlorine is that it doesn’t stop there. Chlorine continues to oxidize the nitrogenous organic matter in the dead bacteria (and dead algae) over time.

As a result, a water treatment facility may have to add 3 parts per million chlorine to achieve a chlorine level of 0.5 parts per million in lines miles away from the treatment plant. So, depending on your location, you can have widely varying levels of chlorine in your water.

photo of a tropical fish Nandopsis grammodes
Nandopsis grammodes

Toxicity of Chlorine

Chlorine is a bad actor in the aquarium. A journal article is pertinent (“Effects of Residual Chlorine on Aquatic Life”, William A. Brungs, 1973):

“Increased use of chlorine and recent studies of residual chlorine toxicity in aquatic systems have emphasized the need for close scrutiny of present disinfection procedures. This review discusses chlorine uses and chlorine chemistry and emphasizes toxicity studies in the field and in the laboratory. Interim criteria, based on knowledge to date, for permissible concentrations of total residual chlorine are: (a) in areas receiving wastes treated continuously with chlorine, not to exceed 0.01 mg/l for the protection of more resistant organisms only, or not to exceed 0.002 mg/l for the protection of most aquatic organisms; and (b) in areas receiving intermittently chlorinated wastes, not to exceed 0.2 mg/l for a period of 2 hr/day for more resistant species of fish, or not to exceed 0.04 mg/l for a period of 2 hr/day for trout and salmon. If free chlorine persists, more restrictive criteria are warranted.”

The pertinent data point here is the 0.04 mg/l limit for trout and salmon. This is 0.04 parts per million (ppm), or 40 parts per billion. This is a very tiny amount of chlorine, especially considering most water is chlorinated to a level 25 to 100 times heavier than that.

Image of a tropical fish Nandopsis Tetracanthus, Biajaca, Cuban Cichlid
Nandopsis Tetracanthus, Biajaca, Cuban Cichlid

Another reference is pertinent (“Toxicity of Combined Chlorine Residuals to Freshwater Fish”, John Zillich, 1972):

“On-site continuous-flow bioassays on fathead minnows using chlorinated and dechlorinated effluents from two treatment plants have shown residual chlorine to be the principal toxic agent in these effluents. The toxic effects at both locations were similar, in spite of the fact that one plant received metal-finishing wastes and the other did not; in both cases, the pH and dissolved oxygen concentration were favorable for fish life. The residual chlorine concentrations that produced a 100 percent kill was 0.16 and 0.21 mg/l; threshold concentrations were 0.04 and 0.05 mg/l.”

Fish started dying at a concentration of 0.04 to 0.05 ppm (mg/l). This is one tenth the minimum chlorine recommended by the EPA for safe drinking water. This is why forgetting to neutralize the chlorine in a water change can kill fish in minutes.

Zillich goes on to say:

“McKee and Wolf, in their discussion of chloramines, stated that chloramines were more toxic than free chlorine to warm-water fish. As little as 0.4 mg/l can kill adult fish, and 0.05 mg/l is lethal to trout fry. Coventry et al. reported that the average chloramine concentration of 0.76 mg/l was fatal to hardy minnows and that an average concentration of 0.4 mg/l was instantly fatal to sunfish and some bullheads, and a maximum concentration of 0.06 mg/l was fatal to fry after 48 hours….

Merkens stated that the toxicities of the chloramines and the free chlorine must be of the same order. The log median survival time of rainbow trout was directly related to the total residual chlorine. At pH of 7, 0.08 mg/l of residual chlorine almost all in the form of monochloramine, killed about half the test fish after 7 days of exposure.”

So both chlorine and chloramine are toxic in extremely small quantities.

tropical fish image Geophagus camopiensis, Rio Aporema_
Geophagus camopiensis, Rio Aporema
“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. They will invariable be “newbies” with newly or poorly set-up aquariums, probably with cloudy water. Chlorine reacts rapidly with any living matter in the water. Add chlorinated water to an aquarium with a couple of billion or trillion bacteria and other organisms floating in the water and a small percentage of the organisms will die and the fish will live. But just wait. When (and if) the fish enthusiast “gets it right” with clear water, their fish will die.

Chlorine is Capricious

This reactivity of chlorine with suspended organic matter and bacteria in the water column in an aquarium gives another peculiarity of chlorine. It affects fish very capriciously. Sometimes only one fish dies, sometimes a few fish die, and sometimes most of the aquarium dies. It depends on the concentration of the chlorine, the concentration of the suspended organics, and where the fish are in the aquarium.

This capriciousness also manifests itself over time. Sometimes if the chlorine dose is high the fish die very rapidly. But sometimes the fish just get stressed for oxygen due to damage to the gills from the chlorine. The fish get into the most oxygenated water they can, typically near the surface. Then they may slowly die over a span of a week or so. Sometimes some will make it through. It’s not a clear cut “all die at once” scenario. A research article is pertinent:

Some Histopathological Aspects of Chlorine Toxicity in Rainbow Trout Mahjoor and Loh, 2008

“Eight-month old rainbow trout (Oncorhynchus mykiss) were found dead in a freshwater recirculation system in the key center of fish health research center of University of Tasmania, Launceston, Australia. Ten days prior to this incident, the system was shut down during a blackout. Fish were found swimming in a swirling motion or died suddenly. Ten days later mortality rates reached 60%. The fish were immediately fixed in 10% formalin buffer and were sent to DPIWE diagnostic laboratories in Launceston. Investigations revealed that the aquariums had been refilled with town water that had variable levels of chlorine and even after sodium thiosulfate was added, chlorine levels were still as high as 0.2 ppm. The type of Chlorine used by the water department was Chloride dioxide and CL 2 (Liquid).

Dead fish had enlarged gall bladders. Mild multifocal dermatitis was observed in the skin. Their carcasses appeared pale and anemic. Wet preparations of skin scrapes and gill biopsies were negative for parasites.

Histopathological examination revealed gill edematosis and foamy vacuolation especially in the proximal part of the secondary lamellae. Gill epithelial cells were swollen. This feature was more prominent in chloride cells. Epithelial lifting and telangiectasis in the secondary lamellae were also found.”

As usual some interpretation is necessary. At very low concentration of chlorine some fish died suddenly and some exhibited twirling before they died. Others died over a span of ten days due to damage to the gills and internal organs which resulted in swelling of the gills and organs. Symptoms included peeling and patchy reddening of the skin.

chlorine burn in fish
chlorine burn in fish

This is quite normal in the aquarium also. Some fish die rapidly, others take a few days, and some sometimes survive. This can be capricious and sometimes only one species of fish will die while the others live. Sometimes when several tanks have a water change at the same time one tank will be completely killed while another will be just fine. That’s just the dynamics of chlorine in a system filled with various organic materials.


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