Reef Tank Evaporation Rate – How Much Will You Lose?

If you are currently keeping a saltwater tank, there is one thing you can count on. Losing water due to evaporation. If you haven’t noticed yet, the water level in your tank will be reduced every day due to water evaporation.

reef tank water evaporation

The problem with water naturally evaporating from your tank is if you don’t replenish it, your tank will become more concentrated and your salinity level will rise.

Water evaporates but salt and other elements do not. It’s important to only use fresh water to replenish what has evaporated.

The rate of water evaporation will vary but you can expect to lose around 1/2 gallon of water per day if you have a 50-gallon saltwater tank.

Let’s look into this topic further.

How much water to expect to lose every day due to evaporation?


As mentioned, the amount of water that will evaporate from your tank can vary. The following items all play a role in determining the rate of evaporation.

  • Tank dimensions/ larger surface area
  • Open top vs closed top
  • Water movement
  • Airflow
  • Temperature/humidity

These are all are factors to consider. My 120-gallon reef tank has the dimensions of 60″ x 18″ x 25″ and it evaporates water at a rate of 1-1.5 gallons per day. 

Smaller tanks may not evaporate as much water. I have also talked to people that plan on 1% of their tank’s water volume to evaporate. 

So if you have a 100-gallon tank, you may need to replenish 1 gallon of water every day. This is just a rule of thumb but it can be somewhat accurate. 

Why does water evaporate so fast in a saltwater tank?

One of the main reasons is the temperature/humidity and time of year.

Water can evaporate from your tank at various rates throughout the year. Most people find that in the winter when the air is cold and dry, water will evaporate much faster than in the summer.

You might be losing 1 gallon per day in the summer, but when winter rolls around, you might lose 1.5-2 gallons per day. 

Open top vs tank cover

Having an aquarium with an open-top ensures your water will evaporate faster. You will notice a considerable difference between an open top and a tank with a closed lid or glass top. 

Larger surface area/tank dimensions

Tanks that are larger in size with more surface area will evaporate water at a faster rate. My 32-gallon Biocube that has a full lid, small surface area, only evaporates about ½ gallon every few days. My 120-gallon open-top aquarium is much larger and will evaporate about 1-1.5 gallons per day. 

Water movement/Airflow

If your like me and like plenty of water movement in your tank, you will notice that the water will evaporate at a higher rate. 

If you are using fans pointed at the water surface for cooling, you will notice a higher rate in evaporation. Plenty of airflow directly affects the water evaporation rate.


Depending on where you live and time of year, this will have a direct effect on the water evaporation rate. If you live in a very humid and warm area, you will see larger volumes of water evaporate versus if you live in a cooler and dry area. 


High-intensity lighting such as T5 or metal halides will create a lot of heat and water will evaporate faster versus tanks that are using LEDs or lighting that doesn’t create as much heat and humidity. 

How can you slow down the evaporation rate?

There are a few things that you can do to help slow down the rate of water evaporation. 

  • Add a top to your tank. Glass top lids are great and will make a huge difference. 
  • Reduce the tank temperature if you are at a higher range of water temperature, you will notice a difference if you can bring the temperature down slightly. Make sure to stay within the proper range for your livestock.
  • Reduce the light time. The longer your lights are on throughout the day, the more water can evaporate. 
  • If you can control the humidity level in your home, consider reducing it. 

How to top off evaporated water – refilling your tank

There are 2 ways that you can replenish your evaporated tank water. Manually or automatically. If you have a larger tank that has a high rate of evaporation, you may want to consider purchasing an auto top off device.

Any sized tank can use an auto top off, but people with larger tanks will appreciate having them. 

If you don’t have an ATO, you will have to do this manually. It’s very important to only replenish your tank with RODI or distilled water. Never use tap water. If you know that your tank will evaporate 1 gallon per day, you can make your water in advance and add the 1 gallon back into your tank each day.

It’s quite simple to store a 5-gallon jug of water close to your tank with a small container to scoop the water out and add it into the display tank. 

But of course, if you can automate this process, not only is it way more convenient, but it is better for water stability as well. 

An auto top off system will keep your salinity level consistent by replacing the water in your tank as it evaporates. Once your water reaches a certain level, the ATO will kick in to replace it. It’s difficult to maintain water stability without an ATO as it’s possible you could forget to do the daily top-offs or if you are away from your tank from any length of time, the water will not get replaced.

Check out my review of the best auto top off systems here.

Water evaporation calculator

Now that we know the water will evaporate at different rates based on temperature, humidity, air movement, and surface area, it would be great to have a calculator to determine the evaporation rate. 

Here is a simple calculation to help.

First mark on your aquarium where the optimum water level should be. If you don’t top up your water for 7 days, quite a bit of water will have evaporated and your water level will be lower. 

Measure the distance between the original mark (normal water level) and the new mark (evaporated water level). Use a tape measure. 

Take this number and multiply it by the length and width of your tank (in cm). This will tell you how many milliliters have evaporated over 1 week. 

For example: Let’s say your aquarium has a volume of 208 liters. Which is a standard 55-gallon tank. The dimensions are 122 cm (length) x 33 cm (width) x 53 cm (height). 

Over a period of 7 days, you measure the water has dropped by 2.5 cm or 1 inch. 

Take 2.5 x 122 x 33 = 10065 ml. Just over 10 liters of water evaporates each week.

To calculate the daily rate:

10065 / 7 = 1438 milliliters or just under 1.5 liters per day.  

Aquarium evaporation covers

As mentioned above, using an aquarium cover can really help slow down the rate of water evaporation. There are a few different types of covers available.

  • Glass lids
  • Plexiglass covers
  • Canopy hood
  • DIY covers

Glass lids are the most popular, as most aquarium manufacturers sell the proper dimension covers for each tank. They are not expensive and easy to find. 

If you have a complete aquarium kit, most likely it will come with a full hood canopy that is designed to fit it. 

If you are into DIY, you can create a lid that will fit onto your tank. There are many options available.


Remember that all saltwater tanks will encounter water evaporation. There is no way around it. So it’s important for us to maintain consistent water parameters in our tank at all times and this includes maintaining a proper salinity level.

We can do that by manually or automatically replacing evaporated water every day. I recommend that you automate this process with an auto top-off system. It will not only maintain water stability but it can reduce the amount of maintenance that you have to perform on your tank which is a great thing.