South African Tapwater

Author: Dr. L.E. Freese

Is our tapwater dangerous to pond and aquarium life?

It is very important to maintain superior water condition in an aquarium or pond, as this is the living environment of the fish. If the water is unsuitable, the fish become stressed which leads to ill health or even death.


Although chlorine is considered safe for human consumption, it is deadly to fish.

The concentrations of chlorine in South African waters vary from area to area, and therefore it is best to contact your water supplier and ask them what they use to sanitise the water in your area. The average concentrations of chlorine in the listed cities are as follows:

Bloemfontein:     0,1-0,2 mg per litre

Cape Town:        0,1-0,4 mg per litre

Durban:           0,97 mg per litre

Johannesburg:     0,2-0,8 mg per litre

Pretoria:         0,1-0,3 mg per litre

These concentrations are very dangerous to fish, as some scientists say that concentrations of as little as 0,05 mg per litre are harmful. The chlorine causes damage to the lining of the gills of the fish which in turn decreases the ability of the fish to absorb oxygen. Chlorine kills the nitrifying bacteria (good bacteria) in the bio-filters and after 3-5 days there is an ammonia spike, which leads to ill health of the fish.


Chloramine is added to water instead of chlorine due to its stable nature and therefore stays in the water for longer. Chloramine is not used in all the treated water and therefore it is best to contact your water supplier for more details. Chloramine is becoming more popular as a water treatment method in South Africa. Chloramine is dangerous to fish in that it passes through the membranes of the gills and into the blood stream where it binds with the iron in the red blood cells. This prevents the blood from transporting oxygen, which leads to lethargy and eventually death.

An important fact to note is that chloramine is not removed through reverse osmosis, as it is able to penetrate the membrane. Chloramine breaks down into chlorine and ammonia.

Established bio-filters are capable of removing the ammonia but in new systems, zeolites will have to be used.


Heavy metals enter tap water in numerous ways: lead and copper piping, air and ground contamination and metals that are in contact with the aquarium water itself (filter pumps and metal piping).

Intoxication of heavy metals in fish is not detected early, as the intoxication occurs with the accumulation of heavy metals in the water and in the tissues of the fish. Here they interfere with the physiological processes of the organs. The fish’s health deteriorates and bacterial or fungal infections are common.

When choosing which dechlorinator or water conditioner to use, it is important that it removes chlorine and chloramine, and neutralises heavy metals. Water conditioners that contain additional ingredients that assist the mucous layer and stimulate the healing process should be considered to be superior as they protect the fish from infections. Some artificial mucous coatings can cover the gills leading to a decrease in oxygen absorption.  

In general South African tapwater is safe to use in ponds and aquariums provided that it is treated appropriately.  

May 2001