Evaporation in Your Aquarium: Process, Causes, and How to Reduce It!

Evaporation affects anyone with an aquarium. All tanks no matter how large or small need topping off to replace the water. This is especially true with reef aquariums and tanks that use a filter sump. If you’ve ever left your aquarium unattended during a trip away from home, you know how much the water level can drop. Aquarists using a filter sump have to be especially careful since evaporation can quickly lower the water and expose the return pump. We’ll take a look at how the evaporation process works, how to slow it down and what to do to keep your water level up in your aquarium.

What is Evaporation?

Evaporation is the process by which water changes from a liquid to a vapor. It’s how clouds are formed. The oceans, lakes, and rivers lose water to the atmosphere. Eventually, the water returns to the earth in the form of rain and snow. The same thing happens with our aquariums; water is lost to the air in the room, except it never returns to the aquarium. We have to add water back to the tank. Evaporation involves a lot of complex physics, but we won’t get into those calculations, however, it does help to understand the evaporation process and how it affects your tank.

Evaporation occurs at the surface of the water. The more exposed surface area, the greater the potential for evaporation. Water loss occurs when a water molecule gains enough energy to break free of the liquid state and enter the air as vapor. But how do water molecules get “energy”?

Warm Water

Water molecules are always bouncing around and colliding. They move faster as the water temperature increases. As the molecules are energized by heat, they “hit harder”, gaining enough energy to shift from a liquid to the gas phase. Water temperature is one of the factors controlling the rate of evaporation.

Air Movement

Blowing air across the surface of the aquarium sounds like a good way to push heat from lighting away from tank. It is, but there is a tradeoff to consider. Air movement across the water's surface carries away the rising water vapor, making it easier for more water to evaporate. Evaporation slows down as the humidity in the air above the water surface increases. That’s why adding a glass lid to your tank reduces the evaporation rate. The vapor layer in between the water surface and the glass lid makes it harder for evaporation to occur. Some water condenses on the glass lid and drips back into the aquarium.

Water Movement

Certain types of water movement can increase the evaporation rate in your tank. For example, Gyre pumps provide tremendous water flow throughout the aquarium. But some aquarists see a dramatic increase in evaporation. This is because the wave-like ripples at the surface create a greater surface area for evaporation. Air stones used to blow off carbon dioxide and raise pH will also increase water movement and surface area.

Tips to Reduce Evaporation

It would seem that everything needed to keep a reef aquarium, like water movement, circulation pumps, sump filters, skimmers and open-top tanks contribute to evaporation. This is true but it doesn’t mean we shouldn’t use these valuable tools. The idea is to know how they contribute to the evaporation rate. It may be helpful to mount your Gyre or flow pumps a little lower under the water surface. If your protein skimmer causes turbulence in the sump, try placing a sponge block or similar material to reduce the “splash” from the outflow. If your tank runs hot due to lighting, room temperature or pumps, it may be necessary to install a chiller.

Technology to the Rescue

It’s not possible to completely eliminate evaporation. You can make minor adjustments, like decreasing surface agitation, but reef tanks need water movement, lighting and filtration. Many aquarists like the light penetration and view that an open-top aquarium offers. These are essential for reef-keeping. But there is a way to combat evaporation! If you’re tired of adding water every day, an automatic top off (ATO) system is just what you need. The full system consists of a water level sensor, water reservoir, and a small pump.

An ATO automatically senses when the water level in your aquarium or sump is too low. The ATO turns on the pump, adding RO water back to the aquarium. The sensor switches the pump off once the correct water level is reached. ATOs are safe and reliable. They even have safety sensors that prevent over-filling. An ATO eliminates the need to add water every day. Just keep the reservoir filled and the ATO does the rest. Many reef aquarists like that an ATO prevents salinity swings because water is added as needed, not all at once at the end of the day. No matter what size aquarium you have, there’s an ATO to match.