While salinity and other water parameters are relatively constant on the natural coral reefs, they’re highly prone to undesirable fluctuation in the closed system of a marine aquarium. As conscientious hobbyists who want to provide the most naturalistic environment possible for the sensitive fish and invertebrates in our care, one of our primary responsibilities is to maintain stable water-chemistry values in the face of various influences that tend to promote instability.
When it comes to maintaining stable salinity (or specific gravity, depending on what you’re actually measuring) in a saltwater system, you’ll find there are several factors at work to undermine your efforts. Here’s how to counteract them:
#1: Top off for evaporation with fresh water only
Remember: salt and other dissolved solids do not evaporate. When evaporation occurs in your aquarium, the salinity actually increases as the water volume decreases. Thus, if you top off your tank with salt water to compensate for evaporation, the salinity will climb because now you have more dissolved salt than you began with. On the other hand, replacing the evaporated water with an equal volume of purified fresh water will keep the salinity right where you want it.
A key phrase in that last sentence is “an equal volume of purified fresh water.” For the sake of stability, it’s important that you replace no more or less water than was lost to evaporation. To ensure that you’re adding the correct volume at each top off, it’s helpful to mark the desired water level on your aquarium or sump in some manner. For example, I simply place a piece of masking tape on the sump of each of my systems at the water level I want to maintain.
Also, try not to allow too much time to elapse between top offs. The longer you wait, the steeper the fluctuation in salinity will be. Freshwater top offs should be considered a daily chore.
#2: Match salinity when mixing new salt water
Of course, any replacement salt water you use for water changes must be of the exact same salinity as the dirty water you’re removing. But don’t rely on the measurement you get immediately after mixing new salt water. Let the water sit at least overnight while it’s being heated (to match the temperature in your tank) and aerated, then test it again before using it. Sometimes you’ll get a slightly different reading after the water has had a chance to mix and stabilize, in which case you may need to make slight adjustments by adding either more salt or fresh water before using it in your tank.
#3: Make one-to-one water changes
Just as you need to compensate for evaporation by adding an equal volume of purified fresh water, any time you perform a routine water change, it’s critical to replace no more or less salt water than you removed. To achieve this, you can use that same mark on your aquarium or sump as a reference for how much salt water to replace (assuming you’ve already topped off the tank that day) or you can simply keep track of how many buckets, etc., of water you remove and replace the same amount. If you use the latter technique, you must also be sure to fill the bucket or other vessel to the exact same level each time.
#4: Compensate for salt creep
One of the most visible factors influencing salinity is salt creep—that crusty salt buildup that occurs on any surface exposed to air and saltwater spray, such as the top edge of the tank, the cover, power cords emerging from the aquarium, etc. Over time, this loss of salt can lower the salinity, so you may occasionally need to compensate by adding small quantities of sea salt (dissolved in aquarium water and added very gradually) to your system.
#5: Don’t forget the small stuff
In addition to freshwater top offs, water changes, and salt creep, there are numerous smaller factors that we don’t tend to give much thought but can still have an effect on salinity over time. For example:
- Whenever you acclimate a new specimen, some salt water is necessarily removed from your system. If you forget about this and later top off the tank with fresh water, you’ll end up lowering the salinity in the process, if only to a small degree.
- If you get into reefkeeping, a similar example would be bagging up coral frags to sell to your local dealer or fellow aquarist. This often involves the removal of small amounts of aquarium water that would need to be replaced with an equal volume of salt water.
- Your protein skimmer can even have an effect on the salinity level. Each time you empty your collection cup, you lose a small amount of salt.
None of these minor factors taken singly is going to cause a sudden, precipitous shift in salinity, but over time, they can have an impact. Regular testing with a refractometer or hydrometer—a routine maintenance chore that should be conducted on at least a weekly basis—will reveal any shift away from the desired salinity/specific gravity level, so you can react promptly to correct the problem before it becomes too pronounced.