Can I Convert My Freshwater Aquarium Into a Saltwater Aquarium?

by Britt
Photo credit: Vojce /

Freshwater aquariums are often the starting point for those new to fishkeeping. It’s a lower-cost entry point and generally more forgiving than saltwater aquariums as you learn the ins and outs of maintaining your tank.

But what if you started with a freshwater aquarium but are now ready to switch? Must you purchase everything from scratch to set up a new aquarium? It’s a question that we hear often. The good news is that you CAN convert a freshwater aquarium into a saltwater aquarium with some planning, preparation, and a few key steps.

This article will discuss the pros and cons of having a saltwater aquarium and some other essential information to help you decide if this is right for you and your fishkeeping journey.

If you are ready to make the change, we’ve also included a step-by-step guide to switch your tank over properly. Let’s get started…

Can You Put Saltwater Fish in a Freshwater Aquarium?

If the reason you are considering changing to a saltwater aquarium is a love of the bright, colorful fish, you may be wondering if you can simply place the new fish in your current aquarium setup.

Spoiler alert: Don’t do that!

Saltwater fish have adapted and evolved to thrive in environments with a high salt concentration. This means changes in their bodies at a cellular level and how their bodies process water to eliminate the excess salt.

When placed in freshwater, they cannot adjust to the new environment. As they take in the water, their cells absorb it through osmosis, causing them to swell. Ultimately, this is a fatal mistake. If you want your saltwater fish to survive, let alone thrive, you must ensure that they are in a saltwater environment.

Is a Saltwater Tank Harder to Maintain than a Freshwater Tank?

The most common reason people are reluctant to fully switch to a saltwater tank is the work and expertise required to grow and maintain the aquarium. To avoid sugar-coating it or misleading anyone, yes, there will be more work to keep a saltwater tank. However, it isn’t as difficult as you may have been led to believe.

The most significant difference is the importance of maintaining water conditions. Freshwater fish in the wild are generally found in smaller bodies of water, while saltwater fish are in larger ocean settings. The bigger the body of water, the more consistent the water conditions remain.

Freshwater fish have evolved to be more forgiving of fluctuating water conditions to survive the impact of weather changes and environmental shifts they may face in the wild. Therefore, they are hardier and better suited for those who may struggle to maintain water parameters. This makes a freshwater aquarium easier to keep because it is more forgiving of small changes or mistakes you may make along the way. At the same time, a slight shift could set off a series of issues in a saltwater tank and could devastate the ecosystem you’ve built.

To further complicate things, there are more parameters to consider with a saltwater aquarium – the most obvious being salinity. This means more testing, tracking, and adjusting to maintain optimal conditions.

Do I Need a Special Filter for a Saltwater Tank?

The good news is that most filtration options used in freshwater aquariums are also safe for saltwater aquariums. However, most saltwater setups require more filtration, meaning you may need to supplement it with a second filter like a trickle filter. The one exception would be an under-gravel filter, as the preferred aquarium substrate for saltwater is sand versus the gravel those filters are designed to work with.

If you are making significant changes to the aquarium setup, like a new, larger tank, this is an excellent time to step back and reassess the filtration that you are using.

What Are the Pros of a Saltwater Aquarium?

We’ve touched on the fact that saltwater aquariums can be more challenging to maintain, but there is much more to consider when deciding on the right setup for you and your lifestyle. Those who love having a saltwater setup in their home will happily share the following pros:

  • Saltwater fish and plants are often brighter and more vibrant in color, creating a more eye-catching underwater scene for you to enjoy.
  • You can create unique and visually appealing reef aquariums with anemones and coral reefs.
  • Saltwater aquariums are constantly changing, growing, and evolving, adding ongoing interest. For example, new inhabitants in your aquarium may come from live rock or sand you introduced without realizing the inhabitants were also part of the mix.

What Are the Cons of a Saltwater Aquarium?

Of course, like every decision we make as fish keepers, there are also some cons to consider. While I don’t share these to discourage you from changing to a saltwater setup, I believe they should be factored into your consideration to make an educated decision. The cons include the following:

  • Saltwater aquarium setups are more costly in the initial setup, cleaning, and other ongoing care.
  • Effective maintenance means stable water conditions with little to no fluctuation, making mistakes and accidents that could cause illness or disease more likely.
  • Maintenance requires testing and tracking many parameters, including temperature, salinity, pH, KH, ammonia, and nitrates. This will take more time on a regular basis.
  • You will need a larger aquarium to accommodate the same number of fish. This means that if you are planning on changing over your tank, you will be limited to a smaller number of fish.
Photo credit: Tatiana Gordievskaia /

How to Change from a Freshwater to a Saltwater Aquarium: Step-by-Step

Step One: Clear Out Your Tank

Before you can take any steps to set up your new saltwater aquarium, you will need to ensure that any elements of the freshwater tank, such as freshwater fish or plants, have been removed and dealt with accordingly. You may be able to pass any living inhabitants onto a friend or fellow fish keeper or speak with your local pet store. Many stores will accept healthy fish in exchange for a store credit that can then be used to purchase the additional supplies you will need for your new saltwater setup.

Step Two: Thoroughly Clean Your Tank

After all living species have been removed and rehomed, you can then turn your attention to the tank, gravel, decorations, and other accessories. Remove everything from the tank, scrub it carefully, and disinfect it for your new setup.

An effective and natural solution for cleaning and disinfecting the tank is a 1:1 mixture of vinegar and water. Another option is to clean the tank carefully with bleach. This is safe to use when done correctly. When mixing your bleach and water, never use a bleach concentration higher than 10% (9 parts water to 1 part bleach). Avoid mixing it with any other cleaning chemicals or solutions.

The vinegar and bleach solutions can also be used to clean your tank accessories. You can discard any gravel substrate, as you will set your tank up with sand instead. For decorations, consider your new setup and whether they will fit in with your new display. For example, if you are setting up a reef aquarium full of vibrant saltwater plants and coral, you won’t need artificial plants or decorations from your freshwater aquarium. These can likely be sold to recoup some of your costs.

Step Three: Start Set-Up with Your Substrate

Unlike the gravel often seen in freshwater aquariums, saltwater aquariums usually use aquarium sand or crushed coral. These reflect the surface that saltwater fish would see in the wild, which helps to support their unique needs.

For example, some fish, like saltwater Gobies, feed by picking up mouthfuls of sand and filtering out their food particles before spitting the sand back out again through their mouths or gills.

Step Four: Select/Upgrade Your Tank Accessories

While some tank accessories from your freshwater aquarium setup can be reused, this isn’t true for all. As previously mentioned, there is a good chance your current filtration can be reused unless you used an under-gravel filter (UGF). Saltwater tanks do require more filtration than those with a freshwater setup. If your old filter was a basic single hang-on back filter, you can supplement this by adding a trickle filter. A canister filter is an excellent solution for fish keepers who are ready and willing to invest in a new filtration setup.

The standard lighting in a freshwater tank is suitable for a fish-only take but will require an upgrade if you are setting up a reef tank or incorporating live saltwater plants. LED coral reef tank lighting is cost-effective and will work for almost all saltwater aquarium setups. If rated as safe for use in saltwater, your aquarium heater won’t need to be changed.

You will need to pick up a few tank accessories for a saltwater aquarium that aren’t generally used in freshwater tanks. Most importantly, this includes the pumps and powerheads used to create a flow in the tank mimicking the natural wave or current movement in the wild.

Step Five: Add Water

When adding saltwater to your aquarium, you will want to add the salt mix outside the tank. This allows you to ensure that the salinity is ideal before adding it, avoiding a situation where you may accidentally add too much salt and need to balance it. To do this, start with filtered water. Add salt mix and then test the water. Add more salt mix and test again using the previous test results as a guide. Repeat this until the test shows your water is balanced appropriately.

When adding the water, slowly add it to the tank so as not to stir up your substrate too much. But be warned, aquarium sand will often cause the water to look cloudy until it has had time to settle.

Step Six: Accessorize with Live Rock and Other Items

Live rock adds an important visual element to a saltwater aquarium. It also plays a vital role as a natural and biological filter to create a balanced and healthy environment for your fish. After placing the rocks in your tank, it’s important to know that it will take several days for the rock to adjust to the water conditions and your overall aquarium setup. During this time, it is recommended that you test the water daily and adjust, as needed, to establish a proper balance.

This is also the perfect time to add any other decorations you plan to use in your tank. Most saltwater aquariums will not incorporate plastic or fancy artificial decorations. Instead, consider exploring natural decoration options like rocks, driftwood, and coral décor.

Step Seven: Add Live Fish, Corals, and Plants

The final and arguably most exciting step is adding live inhabitants to your aquarium. When purchasing fish, corals, and plants from your local pet store, pay careful attention to the instructions you are given to introduce them to the tank. Identify the more resilient and forgiving species to start with, giving them time to acclimate to your setup before slowly adding more. Don’t rush the process – take your time and set your new ecosystem up for success.

Final Thoughts: Switching to a Saltwater Aquarium Setup

If you have always dreamed about setting up a saltwater aquarium, there is no time better than the present. Many of the items you already own from your freshwater aquarium can be repurposed to help keep costs down, including most filtration options and heaters. But, be prepared for the costs associated with the initial setup, as it is generally more expensive to set up a saltwater aquarium.

Setting up your tank is easy, but it does take time. Try not to rush the process by adding too many things at once or skipping the adjustment time necessary for various elements along the way. Saltwater aquariums are more sensitive to changes in the water conditions. By taking each step, one at a time, testing throughout the process and slowly introducing the various elements to the tank, you can create and maintain a healthy ecosystem that will allow your saltwater fish, plants, and corals to thrive.

Most importantly, take time to enjoy the process! Every aquarium is unique and one-of-a-kind. Allow yourself to create your own special little ecosystem that you and your family can enjoy for years to come.


Britt Kascjak is a proud pet mom, sharing her heart (and her home) with her “pack” which includes her husband John, their 2 dogs – Indiana and Lucifer – and their 2 cats – Pippen and Jinx. She has been active in the animal rescue community for over 15 years, volunteering, fostering and advocating for organizations across Canada and the US. In her free time, she enjoys traveling around the country camping, hiking, and canoeing with her pets.

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