Water Dechlorinator: How It Works and How Much to Use in Aquariums

Many fishkeepers are unclear about water conditioners for aquariums – how they work, potential risks from overdosing, and the differences amongst the many brands of dechlorinators. Based on years of experience with them and the research available, let’s get to the bottom of water conditioners.

Do Fish Really Need Water Conditioner?

Maybe. If your drinking water comes from a municipal water supply or other public water system, then most likely it is disinfected with chemicals like chlorine or chloramine to eliminate bacteria, viruses, and other microorganisms that can cause diseases. These chemicals are toxic to aquatic animals and beneficial bacteria and therefore must be removed from the water using a dechlorinator. Your fish may start to gasp or breathe heavily if they are not given water conditioner.

Your aquariums might not require water conditioner if your water comes from a well, or any other source of water that has not been treated with chemicals. We suggest getting your well water tested to see if it contains any heavy metals because some dechlorinators can help remove them.

Does letting water sit remove chlorine? Yes, chlorine is fairly unstable and will gradually evaporate from water. Chloramine has been used in many water treatment plants as an alternative to chlorine. This is because it is a stable disinfectant that is formed by combining chlorine and ammonia. The water must be dechlorinated to remove chloramine. Chloramine is not easily removed by evaporation. If you are sure your tap water contains chlorine and not chloramine, you can let the water sit for 1-5 days to allow all the chlorine to evaporate. The evaporation process can be accelerated by boiling the water for 15-20 minutes, or using an air stone to aerate it for 12-24 hours. Multi-test strips are used to measure the water.

To inject air into water, activate the surface of the water, and accelerate gas exchange, air stones are connected to an air pump with airline tubing.

What is a Dechlorinator?

The main purpose of water conditioners is to break down chlorine and chloramine and make water safe for fish to inhabit. Almost all dechlorinators contain sodium thiosulfate, which reacts to chlorine and chloramine to form harmless byproducts. Sodium thiosulfate appears like rock salt, or white powder. It is often dissolved with water to create liquid chlorinators. Some water conditioners contain pH buffers, aloe vera to help heal the fish’s slime coats, or extra additives.

Can a dechlorinator remove ammonia from chloramine? Yes, according to their packaging. The main reason for this is because when dechlorinators are used to treat chloramine, they only react to the chlorine part of chloramine and not the ammonia part. Fish are unable to ingest the remaining ammonia ions in the water. Some dechlorinators, such as Fritz Complete Water Conditioner and Seachem Prime, contain additional chemicals that temporarily lock the ammonia into an inert (i.e. ammonium) state for up to 24 hours. The ammonium can then be consumed or further reduced by beneficial bacteria in your aquarium.

All dechlorinators neutralize chloramine and chlorine, but some have additional chemicals to deal with ammonia and nitrite.

Will dechlorinator remove bleach? Yes, dechlorinator will react to the chlorine in bleach to neutralize it more quickly. The amount and concentrations of bleach used determine how much dechlorinator is required. As a starting point, see the directions for neutralizing Purigen chemical filtration media after it has been soaking in a bleach solution.

Is the Dechlorinator harmful to fish?

No. It is not dangerous in most cases. Dechlorinators use oxygen to remove chlorine from water. This reaction can be dangerous in tanks with low oxygen levels. Goldfish and discus aquariums, for example, can need large water changes of up to 90%. If your water has low oxygen content, you can add lots of dechlorinator to further reduce the oxygen. This could potentially cause fish death and harmful bacteria.

To prevent this, most fishkeepers increase surface agitation in the aquariums to increase gas exchange. This refers to the process by which carbon dioxide (CO2) is expelled and new oxygen is added to the tank. However, hobbyists with high tech planted aquariums that inject pressurized CO2 often seek to minimize surface agitation. The intent is to decrease gas exchange so that more CO2 stays in the water for the plants to use. This is in addition to the fact that plants only consume carbon dioxide during the day and then consume oxygen at nights. The dissolved oxygen level in water will drop if it is changed in the morning just as the lights come on. Your aquatic animals could become sick if you add low-oxygen water or a dechlorinator.

What is the recommended amount of dechlorinator per gallon?

Each dechlorinator has its own dosing requirements, so make sure to follow them. Fritz Complete, for example, recommends 1 ml per 10 gallons water. These directions are a bit confusing because different municipalities use different amounts. So how can you determine the correct concentration for your water? Since the dechlorinator manufacturers do not know how much chlorine your town uses, they deliberately make general guidelines that will hopefully cover everyone’s tap water.

Fritz Complete contains an easy to use pump head for dosing 1ml of dechlorinator in 10 gallons water.

How long does dechlorinator take to work? Many companies recommend that you add the dechlorinator directly to tap water in separate containers before adding it to your aquarium. We have never had any issues with the dechlorinator. Instead, we add it directly to the aquarium, then we pour in tap water.

Do you think you have too many dechlorinators in your fish tanks? Fritz complete allows you to dole out up to five times the recommended dose within 24 hours. This is a large range, so there’s a lot of room to make mistakes. Just keep in mind that potent concentrations of dechlorinator will quickly reduce the amount of dissolved oxygen, so it may be best to add an air stone for the next 3-4 hours to increase oxygenation in the water.

It’s a good idea to find out the average chlorine consumption in your community and then do some experiments at home. Let’s assume your town uses 2 parts per million (ppm) of chlorine. If you do a 30% water change on a 100-gallon aquarium and you dose 3 pumps of Fritz Complete into 30 gallons of tap water, does the chlorine test register as 0 ppm? Can you get away with less water conditioner, or do you need to dose more pumps to completely eliminate the chlorine? Bottom line: test your water to use the least amount of dechlorinator you can get away and make sure your fish don’t run out of oxygen.

Use a multi-test strip for quick measurement of chlorine in your water.

Many people ask for our recommendation on the best dechlorinator to use, and honestly, we prefer Fritz Complete Water Conditioner because of the super easy pump head that treats 10 gallons of water per squirt. You don’t have to worry about measuring the correct amount of liquid by carefully pouring it into a cap. Just a few quick pumps and you’re done.