Converting a freshwater aquarium to saltwater

Lion fish in aquarium

A common misconception is that you have to graduate from freshwater to saltwater. This is of course not true, as a freshwater species aquarium can be as challenging as a reef set-up, or as easy.

Knowledge is once again the key to a successful start and the more information you have the more fun you will get out of the hobby.

Another fairy tale is that saltwater requires a larger tank than freshwater. This is not so, a larger aquarium is generally easier to maintain than a small tank. Just picture nature as the ultimate Eco system. Any aquarium you set up is a copy and the smaller the copy the more difficult it gets to maintain and to provide a healthy and balanced environment.

Saltwater aquariums do not have to be expensive either. Despite the fact that most saltwater species are on the more expensive side, it does not imply that you can’t start out with “cheaper” fish.

The more you get into additional equipment the more expensive it gets, but the same applies to a freshwater aquarium.

Clear cloudy aquarium water, remove nitrates and simplify aquarium maintenance

Freshwater and saltwater, including live rock, can be compared by size and cost. The exemption is a reef set-up, where the lighting system alone can easily cost as much as a fish only tank.

As you are fired up by now, we will take a look on what equipment you can use in converting a freshwater into a saltwater aquarium.

The Aquarium

The tank itself can be used as is. There are no special aquariums that differ in fresh or saltwater set-ups. It is advisable to clean the tank with a sponge or filter floss with water only. Household chemicals do contain substances that are detrimental to the system.


The original filtration can be used in the new saltwater aquarium as well. Later on in time, especially if live rock and more animals are added, a change in filter type might be justified. In this case a trickle filter or a protein skimmer can be added.

One exception would be the undergravel filter. As explained further down, sand is the preferred choice of a saltwater aquarium and this kind of filter will simply clog up.

Biological Filtration

The nitrifying bacteria in fresh and salt water are closely related, but slightly different. The freshwater bacteria are good for spiking a brackish tank, but a different strain of bacteria needs to colonize saltwater aquariums. Seeding aquariums with freshwater bacteria will speed up the process of cycling the new saltwater tank nonetheless.

Pumps, Tubing, Heater

The tubing, heater, and air pumps from the freshwater tank can be used in the new saltwater tank. If cleaning is needed, water only is the way to go.

Ornaments & Décor

Plastic plants and décor should not be used in a saltwater tank. Most saltwater fish nibble on everything they can get a hold on. The new decoration can include live rock, rocks, or fake corals in the beginning.


The substrate for saltwater should either be sand, crushed corals, or aragonite. The substrate in saltwaterdoes not only provide biological filtration as in freshwater, but also as a home for many animals that might be added over time i.e. crabs.

Crushed corals and aragonite will keep the pH stable as well as provide calcium for a later update to a reef system.

Should sand have been used in the freshwater set-up, it can be used for the saltwater tank.


At this early stage the regular fluorescent light will be fine. To enhance the color of the fish, one might be replaced with an actinic bulb. Lighting will get more important with a reef tank later on if desired.

Test Kits

Freshwater test kits can not be used in saltwater and have to be replaced by saltwater test kits.

General Notes:

In addition to the above, a saltwater mix and a hydrometer to measure salinity will be required. For the first batch of saltwater, fresh water should be used instead of just mixing salt into the tank. The maintenance schedule for cleaning the tank and water changes remains the same as in freshwater.

For most tanks, all there is to it is to replace the gravel, remove and replace the décor and to add water with the seawater mix.

Most of the additives can be used as well, such as a water conditioner and Algone.

Some additives are specifically made for saltwater, so check the labels on the packaging before using them.

Any products that contain copper must be avoided. Copper is one of the biggest enemies for saltwater aquariums.

After the conversion, the tank will still cycle. But if the existing filtration, with the bacteria cultures in tact, was used, the tank will complete cycling much faster then a new set-up.

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21 thoughts on “Converting a freshwater aquarium to saltwater

  1. Jay says:

    I’m starting up an 80 gallon saltwater aquarium and was just wondering how important is a sump, I have read all types of mix comments about sums are important or you Sony really need it and can get away with a protein skimmer and a canister filter. Also I’m looking towards a reef aquarium. Any help or clarification on this matter would be much appreciated.

    • Thilo
      Thilo says:

      With so many things it really is what you make of it. A sump is a great place to keep all the filters, it increases the water volume, is more easily accessible for treatments/ water changes/ adding equipment, fix or temporary. A sump can also be used to grow macro algae, which help to oxygenize the water among other things. In general I would recommend a sump, if space allows, especially if you plan on establishing a reef setup. It makes things easier.

  2. Phillip Large says:

    Hi there, hoping you can help. Im trying to get my better half an aquarium that can be used for saltwater fish. I’ve done some research and tbh getting more and more confused (this article was the first that helped so thank you).
    Ive been looking at a Juwel Primo 110 tank, would the set up it offers be ok for saltwater -Novolux 80 LED, 100w heater and BioFlow Filter Plus Pump – and if not could u possibly recommend a good start up saltwater tank that would come with everything required?
    Any help would be VERY much appreciated
    Kind regards

    • Thilo
      Thilo says:

      Two considerations in addition. The Primo 110 is 110 liters (30 Gallons) in size, which is fine in general. Now, as far as the start up theory goes, always think ahead and work backwards. What do you want to achieve, how do you envision your tank? What species of fish, corals, polpys, mushrooms, etc. Once you have the “master plan” in mind, you can start up slow and take your time.
      In detail, look at the requirements of the fish you like and the number of overall fish and critters you plan on keeping, this will determine the tank size. If you plan to have a reef setup (corals) you might want to consider a reef ready aquarium. This type is set up for more advanced filtration under the aquarium.
      Either which way you can start at the pace you choose, but you will be able to expand without replacing all your equipment along the way.

  3. Hector M Carrasquillo Sr says:

    I have a fresh water 10GL tank already setup, I would like to start it for Saltwater fish. Is this tank good enough?
    I like to try it out with one fish to see how it goes. Give recommendations please. Thank you.
    I would like to start off with one clown fish.

    • Thilo @ Algone
      Thilo @ Algone says:

      Yes it can be done. To start I would recommend a larger aquarium which would make it easier to maintain a balanced environment. Think 30 Gallons. Either way, you will need sand mixed with crushed corals as a substrate and a salt mix to get started. You can use your filter and lights if you keep it that way. You can also add some pieces of live rock to make it more authentic.

  4. Simon says:

    I’m thinking (willing) to change my tank from freshwater to saltwater, used to have smaller one than I have now,back in my late teens. 54 now so things have probably changed a lot, so could I keep you as a contact and pick your brains please

  5. Nirvan Gurung says:

    I bought a fish tank, with a water filtration and light installed and have been using it for almost one year, I know think it’s time to change to a saltwater aquarium but don’t know if my old fish tank is capable of sustaining saltwater fish. Please help me on this. Can saltwater aquariums be made in any fish tanks??

  6. Ananth says:

    Can we use plastic or artificial plants for decoration in RO purified water ?? doesn’t that harm the fish in the aquarium ?

  7. Ananth says:

    I have a aquarium and as the water supply for both salt water and chlorinated water comes from the same pipeline i dont prefer that water.I use water from RO purifier.Is this safe ? Please suggest .
    And also how many times we need to feed the fishes per day ? If possible do mail ur reply to my mailing address.

    • Scott @ Algone
      Scott @ Algone says:

      RO water is the safest water you can use for salt and freshwater aquariums.

      Our feeding recommendations for fish is not to exceed more then once a day sparingly. Following the instructions on most fish food packaging will almost always result in a significant increase of waste in the tank which can lead to all kinds of problems including algone outbreaks, high nutrient levels, de-oxygeneation. I nature fish don’t have the abundance we can provide in the aquarium and once-a-day feeding is perfectly adequate.

  8. Kevin says:

    I’m encouraged by your post! I’d love to set up my 40 gallon as a salt tank but just don’t have the disposable income for expensive skimmers and power heads just now. On the other hand, I didn’t want to put any animals i’d put in it at risk. There was an ancient old puffer for adoption here and I would have loved to taken care of him but just couldn’t find any sites that would talk about bare bones marine tanks. He’s found a home thankfully but I may still get to try a salt water fish. Thanks much!