Adding Water to Fish Tank

Every freshwater and saltwater aquarium is susceptible to certain amounts of evaporation, especially ones using a canister or hanging filter the water drops significantly and needs to be topped off on a regular basis. What is the best way to add water to a fish tank?

Adding H2O to an aquarium while fish are in the tank is less stressful for the fish
Before you Do:

Check temperature
Pour H2O slowly back into tank

Test H2O with a hydrometer/refractometer.
Salinity=35ppt or 1.0264 specific gravity.
Dilute or add salt
Pour H2O slowly back into tank


The primary reason why most people add water to their aquariums is that the water evaporates. Even while the fish is in the tank, you can safely add more water to it. As long as you don’t agitate your fish, they will not get too stressed about you adding more water.

How to add water to Aquarium with fish in it


Tap water can be a good option for use in a freshwater aquarium if you take a few precautions before you add it to an aquarium. Most municipalities treat drinking water with either chlorine or chloramine for disinfection purposes.

Chlorine is toxic to fish and needs to be completely removed before the water comes in contact with fish. Chloramine is a chlorine chemical that is bonded to ammonia, which is also detrimental to fish. Chlorine can be removed with aeration within about 24 hours, chloramine is much more stable and can be difficult to remove from water.

Chlorine (and the chlorine portion of chloramine) can be removed with the simple addition of a commercial de-chlorinator (follow the manufacturer’s instructions for the treatment amount). While many de-chlorinators remove either chlorine or chloramine, some remove only chlorine, so check the label first



Can I Top Off My Fish Tank With Tap Water


Yes as long as you retreat the tap water. Tropical fish need tank water that is around 25 Celsius which is 77 degrees Fahrenheit. If you add too much cold H2O during water addition or change it can stress them and bring on disease.

Add way too much cold water to a tropical tank and you may even shock and kill your fish. So it’s important when using any water to change water to bring it up to a similar temperature first. 

Tap water contains chlorine that is used to treat municipal water. That chlorine can also kill beneficial bacteria in our filters and harm fish, so we must always add a liquid de-chlorinator (water conditioner,) every time you introduce new tap water to a freshwater tank, especially with fish in it.

A partial water change, leaving all the fish, plants, and decor in the main tank is recommended these days. People in the Aquarium business say that it’s less stressful on the fish.

It’s less stressful for the fish and important biological communities are left intact in the filter and the gravel. A 25% change is a good partial water change.

  • Check temperature,
  • Dechlorinate and
  • Pour slowly back into the tank



How to Prepare Saltwater for Fish Tank


When H2O evaporates from your saltwater aquarium tank, only the freshwater evaporates, and the salt is left behind. In order to maintain a consistent salinity level, you need to top off the tanks with fresh water.

The first step is always to test your salinity before adding reverse osmosis de-ionized water to your tank. You can test your salinity using a hydrometer or a refractometer.

Hydrometers are inexpensive, and great for getting started, however, you should invest in a refractometer and become familiar with using one. Refractometers are easy to use and give you a more precise reading. Take a look at this instrument on Amazon Vee Gee Scientific STX-3 Handheld Refractometer, with Salinity Scale, 0-100, +/-1.0 Accuracy, 1.0 Resolution

Once you’ve tested your salinity and added some water, wait a few minutes and test again, repeat as needed until the salinity is 35ppt or 1.0264 specific gravity. If you are topping off manually you can pour this water directly into the top of your tank. Add water near the flow which quickly mixes the freshwater into the tank. Add salt if needed. 



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