How to Maintain Salinity in a Saltwater Aquarium


In this article, we will explain how to set up the perfect balance of salt in your saltwater, how to accurately test your salt levels, the best way to maintain perfect salinity in your tank and some common mistakes to avoid. We will also recommend a few tools to make the job of monitoring the water salinity an easy and convenient task.

Fish only saltwater tanks salinity levels range from 1.017 to 1.023. Reef tanks range from 1.023 to 1.026. Use a salinity refractometer to accurately test specific gravity (D 20/20) and parts per thousand to ensure accurate aquarium salinity levels. When water evaporates, the salinity increases as the water volume decreases. Salt and other dissolved solids do not evaporate. For best results use pure distilled water to replace evaporated water and when matching salinity levels each time you perform water changes.

Mixing your saltwater

It is best to mix your saltwater in separate buckets. After mixing a salt complex like Instant Ocean, Pro-Reef Sea Salt or Red Sea Coral Pro Salt with pure distilled water or reverse osmosis water you will need to stir until the solids are dissolved and the water becomes clear.

Most reef sea salts are very similar when comparing standard water testing parameters. It is difficult to go wrong however the stability of the water chemistry can vary. If you use a float valve with an auto filling system for your aquarium, then you want stable saltwater chemistry in your backup water reserve for as long as possible. After many months of testing, Instant Ocean, Pro-Reef Sea Salt or Red Sea Coral Pro Salt seem to have the best stability of all the products tested.

Accurately test your salt levels

Test the salinity using a refractometer until target salinity is reached. The Salinity refractometer 0~100 PPT is one of the best salinity refractometers on the market today. With automatic temperature compensation and its chromium sensor housing this refractometer offers two scales of measurement, Specific Gravity (D 20/20) and parts per thousand.

It is reasonably priced and very durable. If you have a saltwater tank this tool is a must have to accurately maintain ideal salinity.

Next place an aerator and heater in each bucket and let stand until the water has reached the aquarium target temperature. You should allow the salt water to aerate, heat and settle for a minimum of 18 to 24 hours then re-test salinity levels before adding to your aquarium.

What is the best water to achieve perfect salinity in your aquarium.

Evaporation can quickly change the concentration of salinity in your tank. Maintaining perfect salinity really takes the daily effort of adding clean water to make up for the water loss from evaporation. There are three standards of water used to fill and maintain your aquarium water quality. They will all balance the salinity in your tank but include varying levels of overall quality.

Expert Tip: Distilled water – This is the absolute BEST solution to maintain the highest water chemistry quality and perfect salinity. Distilled water is the only solution that removes all contaminants to include trace chemicals, fluoride, chlorine, bacteria, viruses and pharmaceutical drugs commonly found in treated municipal water system.

National municipal water tests have consistently shown these chemicals and pharmaceutical drugs to be so small they make it through the best filtration systems on the market and are too small to register when performing Total Dissolved Solid (TDS) testing. Remember that the chemicals and ultra-fine particles too small for filtration systems to capture end up being added to your tank. Over time they can cause your fish and corals to become unexpectedly sick and shorten their overall life expectancy.

The water distiller we recommend is made in the USA from high grade stainless steel for over 45 years of product reliability. The Durastill 30J produces 8-gallons of water per day and features a post filtration system and 4-gallon stainless steel reserve auto-fill storage tank.

We have purchased cheaper foreign made systems and found them to only last a year or two and be made mostly from aquarium toxic aluminum and plastics. We have used our stainless-steel distiller 6 to 9 hours a day since 2008 for our aquariums and it has proven to be very reliable and requires minimal maintenance. Our experience has shown the Durastill 30J is built to last for decades requiring minimal maintenance and repair.

Each gallon of water produced takes about 1000W of power. The secret to our reputation of perfect biologically balanced water chemistry and salinity levels is the use of our water distiller to establish a new tank and for regular water changes. Aquarists who have extensively tested our aquarium water call it “Magic Water”. Now you know the secret to your perfect home aquarium having magic water too.

Pros – The absolute best solution for the cleanest water and to establish the perfect aquarium water chemistry.

Cons – Expensive up-front cost for a quality water distiller. Automatic water distillation more electrical power and time than a reverse osmosis system.

Reverse Osmosis / Deionized (RO/DI) water – Most experienced saltwater aquarist uses RO/DI water to establish and maintain their tank. This is the SECOND-BEST solution providing a cost effective and safe solution for your aquarium. Reverse osmosis filtration systems capture most of the toxins, chemicals and heavy metals, providing clean water to establish and maintain your aquarium salinity. At a minimum we recommend a 4 stage RO/DI system.

Expert Tip: Reverse Osmosis systems do not kill bacteria or viruses without the help of ultraviolet sterilization. The system we recommend is capable of 100 gallons per day filtered through 7 stages. The NU Aqua Platinum series system provides alkaline reverse osmosis water and includes ultraviolet sterilization and a free parts per million (PPM) testing meter.

Pros – Proven results for decades, RO/DI filtration systems are well established and provide high-quality systems essential for creating clean water. An excellent choice to establish and maintain your aquarium salinity. Up-front costs are reasonable averaging $150 to $500.

Cons – Expensive maintenance costs averaging $120 to $210 in replacement filters every year.

Chemically Treated Tap Water

We do not recommend the use of chemicals to “clean” your water. The use of chemical treatments for any aquarium issue should be considered with caution. The gradual side effects of chemical treatment can cause more problems than they solve. Chemical companies promote the chemical sodium thiosulfate, readily available at fish stores under various brands. Sodium thiosulfate neutralizes chlorine instantly.

Pros – It is convenient, easy and doesn’t appear to have any immediate ill effects on fish.

Cons – Sodium thiosulfate is known to depress PH and dissolved oxygen which will stress fish and corals, shorten their life expectancy and may contribute to their death over time.

Common mistakes to avoid when maintaining stable aquarium salinity

1. When you have filled your aquarium and have reached ideal water salinity and chemistry, use a permanent marker to mark the level of the water. As water evaporates you can add pure distilled water or reverse osmosis water to the aquarium until the water level reaches the mark. This will ensure the correct salinity ratio to volume of water with ease.

2. Don’t wait too long between replacing the evaporated water. The greater the evaporation loss, the higher the salinity will grow out of the ideal range. Adding pure distilled water or reverse osmosis water should be part of your daily maintenance routine. Replace no more or less water than was lost to evaporation

3. Salt creep visibly and consistently reduces salinity. It is the crusty salt buildup that occurs on any surface exposed to air and saltwater spray, such as the top edge of the tank, the cover, or anything extruding above the water surface. Over time, this loss of salt can lower the salinity. This often-overlooked cause of salinity loss can be remedied with regular water salinity testing and adjustments by adding small quantities of sea salt (dissolved in aquarium water and added very gradually) to your system.

Pro Tips:

  • 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 reef keeping, 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.

Once you have obtained the perfect water salinity in your aquarium and before you are ready to acquire fish please see our article on Best Fish to Start a Saltwater Tank.

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