How Often Should You Top Off Water In Saltwater Tanks?

There are many different maintenance duties and tasks to perform when keeping a saltwater aquarium.

Sometimes it can be easy to forget certain tasks that are needed to maintain the water quality and health of the tank.

Adding water to your saltwater tank is not only done when completing water changes.

You will need to add water to your tank regularly as it will evaporate over time. 

This is something that often gets overlooked and will have a negative impact on water parameters if not done on a consistent basis.

In this article, I will show you how often you should top off your tank along with some tips that can make the process easy.

Rate of evaporation – How often to top off your tank



The rate of water evaporation will vary based on different factors.

On average you can expect to top off your tank 2-3 times per day with a small nano tank, using around ½ – 1 cup each time. 

Larger tanks will require a top off more often. It’s common for a 120-gallon tank to be topped off 1 gallon or more per day.

I fill my 5 gallon top off container every 4-5 days on my 120-gallon tank.

My ATO will replace the evaporated water several times throughout the day when needed.


Factors affecting water evaporation

  • Water temperature – Warmer water will evaporate at a higher rate.
  • Exposed surface area – Water evaporates faster in a wide shallow tank vs a tall narrow one.
  • Humidity – Water evaporates slower in high humidity.
  • Tank cover – If you have an open-top tank, water will evaporate much faster.

There are some other factors that affect the water evaporation rate which you can read here. 


Why do we need to top off the tank?

The purpose of topping off the tank is to replace evaporated water and maintain a consistent salinity level.

Water naturally will evaporate over time. The rate of evaporation will vary based on the factors mentioned above.

When keeping a saltwater tank, it’s important to know that only freshwater will evaporate and the salt stays behind. 

This is crucial because as water evaporates, the salinity level of the water in the tank will rise.

We always want to maintain our water parameters as stable and consistent as possible. 

A rapid or large increase in salinity can have serious health effects on our fish and corals.

So as water evaporates, we replace it with fresh to keep salinity constant. 

Best ways to top off your tank

There are 2 main ways to top off your aquarium water. 

  • Manual method
  • Use an auto top off (ATO)

With the manual method, you are adding freshwater directly into the tank by hand. Some people will have a specific container to use for top offs or you can make the water ahead of time so it will be available when needed.

It’s good to get into a routine every day where you add the water into the tank at specific times. It would be even better to add the water a few times throughout the day as the salinity will stay more consistent this way.

But who has time for that right? 

Although this is not a huge undertaking, the need to add water on a continuous basis manually can get a bit frustrating.

Before adding fresh water to the tank, always check the salinity level first. This will prevent you from adding too much or not enough water. 

The better method for adding top off water is to use an ATO. An auto top off will automate this process for you so you won’t need to manually do it.

There are advantages to using an ATO system. The biggest one being that the system will replace the freshwater automatically as it evaporates.

The ATO will turn on and off when needed. This is huge for maintaining water stability which is difficult to do manually. 

Best water to use for topping off your tank

The best water to use for top-off is reverse osmosis or RODI water. 

Never replace evaporated water with mixed saltwater. This will increase the salinity to unsafe levels. 


Can you use tap water to top off your saltwater tank?

No, you should never use tap water to top off your tank.

Tap water contains harmful substances like chloramines and fluoride which are toxic to fish.

It also contains nitrate and phosphate which will cause problems with nuisance algae in your tank. 

How to test the salinity level?



Testing the salinity of your water is fast and easy to do. You can use a refractometer or hydrometer but I recommend the refractometer as it is more accurate.

Even more accurate if using a digital one. 

Another option to test salinity is using the Hanna Instruments digital salinity tester. They are very reliable and accurate.

Digital pocket testers and pens are also available which you can buy at Marine Depot. 


Best ATO systems for saltwater tanks

If you decide to go with the automation for top off, there are some great ATO systems on the market that are affordable and work great.

I have used each of these systems and recommend them. Read more detailed info on these ATO systems here.

Tunze Osmolator ATO

Smart ATO

Ice Cap dual ATO


Topping off evaporated water in your tank is a very important maintenance task. Doing this properly will allow for a stable salinity level with healthy fish and corals.

If possible, you should have an ATO system to automate this process for you. They are a valuable and convenient piece of equipment that you will be glad to have.