Using Untreated Tap Water in Aquariums

Reef aquarium
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Do you use water straight from the tap to make-up your sea salt mix or add top-off water to your system? If so, you may be heading for trouble. Even though drinking water goes through our local water supply purification processes, often impurities or chemicals can still be present in the water. Here are some of the potential problems that may arise when using or adding untreated tap water to a saltwater aquarium or reef tank system.

Issues With Tap Water

Most city water departments treat their systems with chlorine and/or add chloramine compounds to prevent potential bacteria outbreaks. The problem here is that:

A lot of dechlorinating products on the market will remove basic chlorine elements, but may not be sufficient enough to break the chloramine bond. Therefore, you need to make sure you are using a dechlorinate that "specifically" states it will remove chloramines.

If the water department fails to maintain the proper levels of chlorine/chloramines to kill bacteria in the water, a bacterial outbreak may occur.


If you are concerned about using your local water supply, you can always have it professionally tested to see what is in it. However, even though the test may come back with good results, there is nothing that says something can't change at the water department at any given time to result in a different test even a day, a week, or a year later.

Phosphates, nitrates, and silicates are elements that naturally occur in water, and it is not unusual for some tap water sources to contain high concentrations of these chemical elements. If these are not removed from the water before using it, you are just adding more of the unwanted elements that give rise to all types of algae blooms in saltwater systems, and that often contribute to persistent problems with higher than normal readings of these chemicals when testing the aquarium water.

Copper and iron are particularly detrimental to reef animals, and these or other metals can often be present in tap water as well.

So what can one do to ensure the freshwater supply that is being used is of the best and safest quality? There is no guarantee that even if you filter the tap water first, problems won't arise. The simplest solution is to learn all about the many methods one has to choose from to purify tap water, or use an alternative water source.