Salty Q&A: Why so Much Emphasis on Evaporation in Saltwater Systems?

An ever-shifting salinity level is very stressful to aquarium inhabitants

An ever-shifting salinity level is very stressful to aquarium inhabitants


I’m a long-time freshwater aquarium hobbyist but considering branching out to a saltwater reef system. In talking with some of my reefkeeper friends, I’ve noticed that they’re rather fixated on evaporation from their tanks and doing daily top-offs. Why so much emphasis on evaporation on the saltwater side of the hobby? This isn’t really an issue at all on the freshwater side. Also, I do a lot of traveling for business purposes, so I’m wondering what others in my situation do about top-offs. – Submitted by Amir”


Great question, Amir! Saltwater aquarists are so focused on compensating for evaporation because they want the salinity in their systems to remain as stable as possible. As water evaporates from a saltwater system, all the salt and other stuff dissolved in it gets left behind. The more water that evaporates, the more concentrated these dissolved solids become—thus the higher the salinity gets. To compensate for this, freshwater top-offs must be performed on a regular basis. However, if you allow too much time to pass between top-offs, your salinity level will constantly rise and fall like a rollercoaster, which is extremely stressful to coral reef organisms accustomed to highly stable conditions.

Another reason evaporation is such a fixation for reefkeepers is that there tends to be more of it going on in their systems than in the average freshwater tank. The robust water movement corals and other reef invertebrates demand, combined with the common practice of leaving the tops of tanks and sumps uncovered to promote optimal gas exchange, promotes a relatively high rate of evaporation.

With respect to your assessment that evaporation isn’t really an issue for freshwater aquarists, I would have to disagree. Perhaps one could argue it’s not quite as significant an issue on the freshwater side, but it’s certainly not inconsequential. Remember, the stuff dissolved in water does not evaporate. That applies to dissolved pollutants as well. So, the longer you allow evaporation to go on without a top-off in a freshwater system, the more concentrated those pollutants will become—likely to the detriment of your livestock.

For some freshwater species, regular freshwater top-offs are critical. Take, for example, African cichlids from Lake Tanganyika or Lake Malawi. Though these are freshwater fishes, they often demand the addition of special salts to their water in order to maintain proper pH and hardness levels. To keep those levels stable, it’s necessary to perform frequent top-offs, just as one must for a saltwater system.

If you do decide to take the plunge into reefkeeping and business travel continues to be an issue, you might consider setting up some sort of automatic top-off (ATO) system. Such systems commonly involve the use of a float valve, which is situated in the sump and connected to a freshwater reservoir (e.g., a heavy-duty plastic container or small aquarium). When the water level in the sump drops to a certain established point, the float valve allows water from the reservoir to flow down into the sump until the desired level is restored, at which point the flow is shut off.

Regular SWS contributor Ellery Wong—no stranger to business travel—discusses ATOs and other aspects of aquarium automation that you might find helpful in his Reefing from Afar series. So be sure to check that out!


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. Great answer Jeff evaporation is a concern in any aquarium.

  2. How much salt do you use in a 20 gallon tank

    • Jeff Kurtz says

      Hi Becky! That depends on several factors, such as the target salinity/specific gravity, the type of salt mix you’re using, etc. Most salt mixes will list the rate on their labeling. If not, a reasonable starting point is about a half cup per gallon of water, but that is a very rough estimate and you’ll likely need to adjust up or down. It’s a good idea to first determine exactly how much salt you need to add to a gallon of water to reach the desired salinity and then extrapolate from there.

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