Fish Tank Water Evaporation: Causes and Solutions

You go for a trip and leave your aquarium unattended for days. You come home and find the water level of your tank has gone down. The fish tank is losing water, but there’s no leak; what could be the possible reason? 

Evaporation! Yes, that’s your fish tank water drying up into the air. Evaporation affects all tanks and aquariums. But is it bad for your fish? Maybe.

Today, I’m going to tell you everything you need to know about how your fish tank is losing water without leaks. This article will help you understand what evaporation does to your tank, why it happens and how you can slow it down.

What is Evaporation, and is It Bad?

When water changes from its liquid form to a vapor, it is called evaporation. This happens to almost all kinds of water bodies like the sea, ocean, river and lake. 

Hot places with less humidity can especially lose water levels quicker. However, the water comes back to earth in the form of rain. Something similar happens with our fish tanks, too, except the water never returns to the tank. 

Evaporation might not seem too dangerous to you, but it can be fatal to your fish if you let it happen for too long. That’s because evaporation leaves behind ammonia and nitrate particles (generated from fish waste).

As the level of water goes down, the level of these gasses keeps rising. When the level of these gasses gets too high, your fish may succumb to it. 

Adding Water to the Aquarium After Evaporation

As a natural instinct, you would want to replace the lost water level by adding some water to your aquarium. This process is popularly called “topping off.” 

Doing this may be okay for fixing the problem for one or two days, but it is not the ultimate solution for fish tank evaporation. It often leads to the increase of nitrate and ammonia levels. 

You should instead try changing the water of the tank when that happens. If you’re planning to change the water every week, change 10% of it.

If you’d like to do it every two weeks, change 25% of the water in the tank. 

Fish Tank Water Evaporation: Causes and Solutions

How to Keep the Fish Tank Cool – List of Methods

Keeping your aquarium cool during the hot summers is one way to avoid frequent evaporation. Here’s how you can do it:

  • Close the curtains of your room and avoid direct sunlight where the tank is kept. Keeping the AC on really helps too.
  • Don’t keep the aquarium lights on for the entire day. Switch it off entirely during hot days.
  • Install a fan to blow the surface of the water and prevent evaporation.

How to Keep Water from Evaporating in a Fish Tank?

While keeping the aquarium cool helps slow down evaporation, there are some more tips I’d like to give you. 

  • Set up your tank right by buying the best equipment and not compromising on the quality of filters. This is a proactive method of preventing evaporation.
  • Change your fish tank’s water filter whenever it needs replacement. A clogged filter can produce heat and heat up the tank leading to evaporation. 
  • Excess heat is the actual cause of evaporation, and if you leave the tank open, evaporation is bound to happen. So, cover your tank with its lid.
  • Measure the accurate temperature of the water and turn down the heater when it gets too hot.

How Fast Does Water Evaporate?

This depends on the humidity and temperature of your room. The speed of evaporation also depends on the setup of your aquarium –how warm or cold the water inside the tank is.

The tank’s surface area, which is exposed to the air, also determines the speed of water evaporation in a fish tank.

If your room temperature is way too hot and the humidity is below 40, you’re going to experience frequent loss of water levels every 2-3 days.  

Role of Filters and Water Evaporation

The filtration system in your aquarium has a lot to do with the evaporation of water. Changing your filter when your fish clog it up with waste and food is a vital step in the well-maintenance of your fish tank.

When this filter continues to function, even if it’s time to replace it, it may start getting hot. 

The excess heat generated from the filter can raise the temperature of the water and lead to evaporation. Hence, it is important that you change the filter before it starts malfunctioning. 

Normal Evaporation Rate in a Saltwater Fish Tank

Water evaporation is inevitable, whether it is a saltwater or freshwater fish tank. The problem with evaporation in a saltwater tank is that the tank will be concentrated with saline if you don’t do anything about it.

It varies how much water you’re going to lose on your saltwater tank, but you can expect to witness a loss of ½ a gallon of water every day if your tank is a 50-gallon saltwater tank. This rate of water evaporation, however, depends on these factors:

  • The surface area of the tank
  • Airflow
  • Temperature/humidity of the room and water
  • Open lid vs. Closed lid

While smaller tanks may not evaporate water as much as bigger tanks, the rate may differ doe every aquarium. 


Does Salt Water Or Fresh Water Evaporate Faster?

You can use either freshwater or saltwater for your fish tank. Freshwater evaporates faster than saltwater because freshwater has just water molecules on its surface, whereas saltwater has both water and salt that take time to evaporate.
The salinity of the water in your fish tank lessens the rate of evaporation.

Does Water Evaporate Faster, Covered Or Uncovered?

When the air above the water surface has more humidity, evaporation tends to slow down. This is why covering your fish tank with a lid is a better option to prevent evaporation than not.

Why Does Water Evaporate at Room Temp?

It is possible that your fish tank water evaporates even at room temperature. This is because there are a few molecules of water that have the ability to gain enough energy to escape from the fish tank to the air.
The water molecules with very high kinetic energy break through the water surface and get evaporated.

Can Condensation Occur in Your Fish Tank?

When water collects as droplets on a cold surface, it is called condensation. Sometimes there can be condensation happening in your aquarium as well.
You are likely to experience bubble condensation on the tank’s lid. This is a result of an air pump or happens due to heat evaporation.

How to Slow the Evaporation of Water?

Cooling down the water or the temperature of your room is the best way to slow down the evaporation of water in your fish tank.
Keeping your tank away from sunlight and placing it in a shaded place can avoid heating up the tank.
In addition, if you’re getting a new aquarium, choose the one that has a smaller surface area because the chance of evaporation increases when water is exposed more to the air.

Can You Use a Humidifier to Slow Down Evaporation?

Yes, you can install a humidifier in your room to slow down the evaporation if your room is too low on humidity.
Dry indoor air during the winter months can cause evaporation of the water in your fish tank. A humidifier can help your house balance the humidity levels and slow down evaporation to some extent.

To Sum Up

I agree that fishes are some of the simplest pets to take care of. But their tanks still need to be monitored to ensure all the elements are working fine.

Evaporation is something that fish owners can’t avoid completely. Since water has molecules that will eventually evaporate, the only way to keep your fish safe from constant evaporation is by slowing down the process.

Turning down the heater, installing an AC in your room, changing filters, etc., are some of the most effective ways of ensuring the tank water isn’t evaporating rapidly.