Evaporation in Saltwater Aquariums

stop Evaporation in a Saltwater Tank

Evaporation from a saltwater aquarium can become frustrating.  Firstly, the tank requires constant topping up and secondly, dependent on the amount, is condensation within the room or rooms of your home where your aquarium is situated.

If evaporated water is not replaced, it is possible that it can lead to loss of life in your aquarium.  How?  Evaporation causes water to disappate and leaves salt behind.  This results in increased salinity in the aquarium’s water which is harmful to both fish and invertebrates.  

In this article I discuss what causes evaporation and how you can reduce it.  Not that you will never be able to stop it altogether.

Factors that affect evaporation

The following factors can increase the rate of evaporation in your saltwater aquarium:

No tank cover

It is well known that open top tanks lose far more water due to evaporation than closed top tanks do.  On a typical open topped tank of 300L, you can expect to loose approx 20 – 25L per week on average, sometimes more.


The heat generated by your choice of lighting can also increase your rate of evaporation.  Metal Halides and T5’s generate a fair bit of heat.  Even LED’s generate a certain amount of radiant heat. 


Fans are generally used in very hot climates to keep the water temperature cooler.  But, by blowing across the surface water of your aquarium they increase evaporation.  

Room temperature

The temperature of a room, especially in hot, dry climates can make the biggest difference when it comes to evaporation.  The other factor that may seem odd to think about at first, is that it can occur more in winter than in summer.  Again this is climate dependent but when the heating is cranked up and the ambient temperature within the room rises, it encourages an increased amount of water loss.  

saltwater aquarium evaporation

How to reduce evaporation

It will not be possible to completely stop evaporation from a saltwater aquarium but the best thing you can do is to add a tank cover.  Covers can be made from acrylic or glass however glass is recommended as it will not bow under the heat of the lighting.

The most important thing to remember is not to completely seal the aquarium.  The reason for this is that this would interfere with gaseous exchange at the water surface (the exchange of oxygen for nitrogen leaving the water).  You should be able to get away with 70% of the tank being covered whilst still maintaining gaseous exchange.

How to replace water lost by evaporation

So, how do you top off a saltwater aquarium?

Here are two main ways reefers top off their saltwater aquariums:


Ensure that your tank to filled to it’s desired level, then mark the water position on the glass with a permanent marker (out of sight is best but it also needs to be easy for you to see where the mark is). 

Keep a watch on the level you have marked and when it drops, add RO/DI water.  Depending on the amount of evaporation, you may need to top off your tank daily.  


Using an auto top off unit will make your life so much easier!  No more wasting time and carrying and lifting heavy buckets to replace the water lost during evaporation.  It will all be done for you! 

In my own experience, I would regard this piece of equipment as essential.  It will automatically monitor you water level and when it detects that the level has dropped, it will automatically off the system from a reservoir.  This can be a purpose made reservoir tank such as a water container however most larger saltwater aquariums have them built in as standard.

Your sole responsibility will be refilling the reservoir which could be once every week or two depending on how big your reservoir is.

An added benefit of using an auto top off unit is you do not have to physically be there.  This means that you can go on vacation* for a week and it will carry on topping off as necessary, as long as you have supplied top off water for the duration. 

*It is always advisable to have someone you trust to come in and check on your tank when you’re away from home for an extended period, just in case something has gone wrong and needs attention.

Can I use tap water to top off my saltwater aquarium?

It is not advisable to use tap water unless you know exactly what is in your water.  There are so many chemicals used in the ‘purification’ process, chemicals that, in large quantities, can be harmful to fish and invertebrates. 

Tap water can also cause algae blooms which can be really frustrating especially for a beginner who is perhaps still trying to get to grips with the basics of water chemistry.  

Read more about the effects of tap water in this article.  

It is recommended that you use RO water for topping off.  (RO/DI water is best)

Does a dehumidifier increase evaporation?

Yes, a dehumidifier can definitely increase evaporation as it’s purpose is to extract moisture from the air. 

Dehumidifiers also give off heat when running and this extra warmth in the room will further dry out the air and cause further evaporation.  

It is recommended that the humidity levels in a room should be between 30 – 50%.  Keeping levels within this range will ensure a reduction in condensation and prevent mould growth.  Although this may cause further evaporation from an aquarium, it will help keep the room at a comfortable level.


Unfortunately evaporation from your saltwater aquarium is something you will have to get used to.  It happens with every aquarium.  

A cover which covers no more than 70% of the top of the tank will help reduce evaporation without inhibiting essential gaseous exchange. 

An auto top off unit is invaluable, saving you time and energy topping off your aquarium automatically.  

Hi, my name is Craig.
I am the owner of The Salty Side.
Firstly, thank you for visiting us. If you have found this article to be informative, and you now have a better understanding of the topic, then my job is done.

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How to Deal with Evaporation in a Saltwater Aquarium
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