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Automating Reef Tank Salinity Control

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Last updated on November 17th, 2023


Automating Reef Tank Salinity Control

Automated Reef Tank

This article about automating salinity control is part of a series of articles I am writing about how to automate a reef tank. In theory, a fully automated reef tank would require you to do absolutely nothing. In reality, any of us that own an aquarium know that is unlikely at best but essentially not possible. That being said, what if we automated 90% or more of the reef tank? Then you would only need to worry about things like cleaning pumps, saving coral that fell over, and other minor jobs.

While each part of automating a reef tank comes with different layers of cost and difficulty, you will need to decide how far you want to take the control. A perfectly controlled environment is expensive and complicated, but one that hits 80-90% may be far less! It's really up to you to decide how far you want to go and how much you're willing to do yourself.

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Reef Tank Salinity Control Automation

Automating Reef Tank Salinity Control is not as important as other parameters, however it can help dial in the "perfect" stable level for salinity. For most reef keepers, this is not going to be a very important topics because a simple Auto Top-Off will handle evaporation and if any small adjustments are needed, a water change can handle the rest.

There are some advanced cases however where controlling salinity actually may be important. One case that I know well because I have to consider it is when farming and selling coral. Any time you sell coral, you are removing salt water from the tank. If you forget to turn off your ATO, you will get fresh water added back in. Leading to a salinity imbalance.

The Simple Case (The 90%)

Let's start with the simple case because that will cover the 90% of reef tanks. When your aquarium water evaporates, the salinity will rise. To bring it back down, and bring your water level back, you just need to add RODI water. For small aquariums, you may do this yourself by pouring a container of RODI water into the sump or tank and calling it a day.

The downside of large top offs is that it causes salinity to go up all day, then suddenly drop all at once. This instability of salt is crucial to think about because that is one of the most consistent parameters in ocean waters. So stability is VERY important!

The solution is a simple Auto Top Off (ATO). You can use a Cheap 5 gallon bucket to hold the RODI water near your reef tank. Or if you want it to look nicer, you can spring for an ATO Reservoir to hold the water for you. Either of these will work, but its all about preference. As far as which ATO to pick, here are a few options ranging in price points.

Simple ATO Setups

Full Salinity Control - ATO Method

If you are looking to have a bit more control on your salinity, you can do this with a reef controller and 2 ATOs. This case doesn't apply for most hobbyists, but anyone selling a reasonable amount of coral may be interested. As you remove saltwater, the ato would replace with freshwater. This would lower salinity and you have to pay close attention through the week or at water change time. This full control would help balance the salinity for you!

WARNING: While this salinity control is a good idea in concept, if your salinity/conductivity probe does not have stable readings, is not calibrated, or generally is not trusted, your tank's salinity will likely be thrown off and possibly kill all reef life. Use this concept at your own risk! I suggest getting VERY high end probes and add safety rails for both high and low salinity in case anything goes wrong.

Salinity Control Setup

What You'll Need

This is actually rather simple. You will set up 2 ATOs. One with RODI water, and one with salt water. If your salinity is frequently low, you may mix the salt water at higher salinity levels like 40ppt. If this is not the case, or your tank is smaller, mix standard levels of salt to maintain the existing levels.

Then all you need to do is turn the outlet controlling the ATOs off based on the current level of salt. My theory is that if the salt level is at or below the level you want, then you will turn off the RODI water and top off with salt water to maintain level. If the salinity is above the desired level, you would turn off the salt water and top of with RODI to help bring the salt back down.

Neptune Apex Example

Outlet for Saltwater ATO

          Fallback OFF
          Set ON
          if Salt > 35.0 then OFF

Outlet for Freshwater ATO

          Fallback OFF
          Set ON
          if Salt < 35.0 then OFF
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Almost seems too easy, right? Honestly, most automated parameters, especially salinity, are VERY easy to control when you separate them out like this. The part that scares people is trying to do everything at once! Remember that when you automate your reef tank, think about solving each part by itself, then bring it together in the end.

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About the Author

Reef Stable Founder John Krenzer

John is a Software Engineer with a passion for saltwater aquariums, as well as the founder and president of Reef Stable. He started in the aquarium hobby as a child with a 20 gallon freshwater aquarium. His interest in aquarium life grew and in 2008, John set up his first saltwater aquarium.

Today, John maintains an over 300 gallon reef tank system, consisting of a 120g reef and a 210g reef. These large tanks are contained within the same system, sharing a sump as a means to reduce total maintenance and increase total water volume.

John writes articles for the blog as a means to learn about more reef aquarium topics. These articles act as a reference for the readers as well as himself. John updates these articles frequently to provide additional information or make corrections as new information becomes available.

If you would like to request an article, tank tour article, or to collaborate, let me know via the Contact Me Page!

About Reef Stable

Reef Stable was initially founded in 2019 as a reef tank parameter log to fill a need. Reef Stable quickly grew, becoming a location to solve all of your reef tank problems as well as a place to learn.

Reef Stable now provides a Reef Blog, Reef Aquarium Guides, Coral Care Guides, Identification and Solutions for Pests and Algae, and Reef Dosing Calculators, in addition to the original Reef Parameter Log.

Reef Stable continues to grow, striving to provide a single location for all your reef tank needs!

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