Converting a freshwater aquarium to saltwater

Lion fish in aquarium
(Last Updated On: March 13, 2017)

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.

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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.

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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49 thoughts on “Converting a freshwater aquarium to saltwater

  1. Dave Armstrong says:

    Decided to convert my tropical freshwater tank to a marine (Fish no corals).
    It’s a 180ltr tank with a jewel 3.0 filter containing sponges.Have been told I can keep my filter so long as I take out the sponges and add something else.can u give me more advise on this and anything else for stage by stage setup.

    • Thilo
      Thilo says:

      You can keep your filter including the sponges, not an issue whatsoever. The conversion is quite simple, just add salt.

  2. Mohamed says:

    Do I need to remove the cover of freshwater tank to convert to salt
    Do I need a protein skimmer for the corals or it is ok with out it

    • Thilo
      Thilo says:

      No, the cover does not need to be removed. The cover prevents too much water from evaporating. A protein skimmer is not needed, but recommended as it filters out impurities that otherwise accumulate. It is a good addition, but not necessary to keep corals in the aquarium.

  3. Kira Koehn says:

    I have a freshwater tank that is already cycled and everything. I had fish in it but have decided to move them to a much bigger tank. So I can’t just add the salt mix to my already cycled tank? Why is that? Do I have to replace all the water that is in my tank or can I do a 50/50 water change and add the salt then?

    • Scott @ Algone
      Scott @ Algone says:

      You need to start from scratch to assure the new water chemistry, including pH and water hardness, is suitable for marine fish. Further, converting from fresh to saltwater will require your tank to cycle again, regardless of the established freshwater nitrifying bacteria. The dying off off your current bacteria after adding salt will likely cause a more severe and toxic cycling process then starting from scratch.

  4. Evan says:

    With LED lighting for saltwater tanks is it the colors or the intensity that is most beneficial to the fish and coral ?

    • Thilo
      Thilo says:

      Intensity is more important for saltwater, fish like it brighter. Color, especially the blue, becomes more important if you go reef.

  5. Tanner Zerfass says:

    I have a 29 gallon freshwater tank haven’t added fish yet but decided I wanted saltwater what do I need to make it so I can do saltwater fish?

    • Thilo
      Thilo says:

      Technically, all you need is to add salt. A hydrometer is needed to measure the salinity of the water. That pretty much is it unless you venture into reef setups, but with that a bigger tank will be helpful. A reef tank requires different lighting a protein skimmer and more appropriate filter and water circulation. You can always upgrade to a reef after you got started with the saltwater aquarium.

  6. Kk says:

    For how long electric equipment need to be keep running eg. Light, sump, power head, wave maker. Is it OK to keep it in for 8 – 10 hrs per day

    • Scott @ Algone
      Scott @ Algone says:

      Everything should run 24/7 except for the lights. Reef tanks will require more light then fish only tanks. A reef requires up to 8 hours of lighting a day. Do not leave light on around the clock though. This will only result in algae outbreaks.

  7. Iraj says:

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

    This is incorrect. The reagents used in both kits are identical. The only difference is that the colours produced from the test are slightly different. If you already have a freshwater kit (the one with bottles, NOT the strips) you can contact the manufacturer and they will send you a saltwater test card. Note that this applies to the #1 manufacturer of these kits in North America. I haven’t dealt with any others.
    Once you go to REEF conditions, you will no longer be able to use the same kit and will need to buy the reef kit.

    • Thilo
      Thilo says:

      In some cases both color charts are included in the test kit. This of course is not the standard. Some kits are not compatible, contacting the manufacturer is advisable to ensure accurate readings.

  8. Emily says:

    i am starting my first salt water tank and i need some advice. It is only a ten gallon but i don’t know what type of fish or lighting (thats cheap) i should get.

    • Scott @ Algone
      Scott @ Algone says:

      As far as fish is concerned, check with your local store for a good beginners saltwater fish. Cardinals and damsels come to mind. Make sure the fish you choose are compatible and suitable for a small aquarium. Regular aquarium lights will do just fine if you are not planning on doing a reef tank. As a beginner, I would recommend a larger tank tank though. 30 gallons and up are far easier to maintain and offer more space for the fish.

  9. Ron says:

    I have a up and running 220 gallon freshwater with 2 fluval fx 6s.can I use the existing water and filters after I add the correct amount of salt.

    • Scott @ Algone
      Scott @ Algone says:

      I’m not sure there’s a question here, but you could add salt to the water in the tank and make sure you hit your target salinity levels before adding any fish. Otherwise, we always recommend setting up the saltwater before adding it to the tank, this includes water used for water changes. Top off water generally does not need added salt, since the salt is left behind as the water evaporates naturally.

      Lastly, we always recommend adding a protein skimmer to any saltwater aquarium. It’s one of the most effective ways to remove dissolved organic waste.

  10. Suma says:

    Hi… Im madly in love with marine fish… but in India cost is too high… a 2 and a half feet tank with set cost you upto Rs.40k.. its too much… my cousin stays near beach and I prefer the natural water than artificial mixing. He will send me half a drum of beach water and salt from that…
    Now I do have douts on how to handle it
    1)my tank is 3 feet, 10mm glass.
    2)if I add beach water into it. Do my fish stays happy?
    3)do i need sump and other stuffs?
    4) I need your help in sorting this out with minimal things with filters ..
    5) what is sump and other skimmer etc…pls give me an idea is it required? If so how to setup. With minimal things.
    6)what is the duration of the beach water that stays healthy in aquarium? When to change it?
    7)if light is too bright (led) is it true that it generates algae into aquarium?
    These agents making me angry with no proper information and guide.. pls help. I donot want to buy anythg from them exceot instruments.

    • Thilo
      Thilo says:

      Ocean water can be used, but there is always a risk of contamination. Technically, no sump or skimmer is needed for saltwater fish only. These are more for reef aquariums. Light intensity has not much to do with algae, it is more the spectrum of the light. Regular LED light will do fine. With the addition of salt, the aquarium is maintained in a same way as any freshwater setup, this includes water changes. Keep in mind that only pure water evaporates, you will need a hydrometer to check and maintain your salinity levels.

  11. Jose says:

    This read was extremely helpful as I have a fully functional fresh water tank and bought what I thought was a fresh water only puffer (GSP). I will have to re home my current tank makes but the puffer and the entire tank will slowly go from FW to BW to SW and maintain 75 percent of its original equipment. I’m thankful I found this read.

    • Jose says:

      To those that might ask, my current set up

      60 gallon rimless acrylic tank running a 300w heater, a fluval above water filter system and a sponge filter dump to a fluval 305 canister filter filtration system.
      At this time I have a rock sediment and basic decorations for the 8 fish maintained. In said tank. Feel free to ask questions as I like to do research.

  12. Isaac Limahelu says:

    I am thinking about converting a 20 gal long aquarium into a saltwater aq. I was planning to only have a small octopus and a few annemonies if possible. I have an Aqueon QuietFlow 20 filter and just a standard heater.
    Any advice or recommendations are greatly appreciated, as I have never had a saltwater aquarium.
    Thank you!

    • Jose says:

      To have an octopus I would suggest a sealed tank. Those guys are escape artists and can easily slip through anything the size of there beak. This being said any filtration system not customed made might leave an opertunity for them to escape.

      I would suggest a hidden filtration system that can be tucked away with only the inlet and outlet showing. This can be done on your own with imagination and some custom piping.

      IMO always get a filtration system that can handle the next size up tank. My fluval alone is made for an 80 gallon tank. Works like a charm in my 60 gallon tank. Sponge filtration systems keep needed bacteria in the tank.

  13. Kara Quinn says:

    I’m planning to convert my 55g freshwater tank to a saltwater setup: thinking clowns, a few smaller angels, some inverts and cleaner crews but no corals. My current tank has an inexpensive Tetra Whisper filter system on it. I can’t imagine that will hold up well enough for marine life so any recommendations on a replacement?

    • Scott @ Algone
      Scott @ Algone says:

      Hi Kara, you should consider a sump with an overflow for your marine tank. The single best filtration device for a saltwater tank is a good protein skimmer. A protein skimmer removes dissolved organic waste before it becomes a bigger nutrient problem. Check here for more information about protein skimmers. Skimmers work best if they are fed surface water, since dissolved organics accumulate there. This is why an overflow feeding a sump is preferred as it directs surface water from the main tank to the sump. Placing the protein skimmer into the sump in turn makes this a very efficient waste removal system. In addition you should also run an adequately sized standard aquarium filter and/or setup the sump to include a particle filter where the water enters from the main tank. A visit to a knowledgeable fish store specializing in saltwater fish should be able to steer you to the right equipment options for your need and budget.

  14. 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.

  15. 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.

  16. 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.

  17. 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

  18. 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??

  19. Ananth says:

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

  20. 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.

  21. 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!

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