Mardel Maracyn®

  • Highly effective treatment for bacterial infections in fish
  • Effective against gram-positive pathogenic organisms
  • Symptoms inlcude clamped fins, swollen eyes, heavy or rapid breathing, patchy coloration, change in swimming behavior

About Mardel Maracyn®

Provides relief from bacterial infections such as body fungus, gill disease and popeye in fresh and saltwater aquariums.

Mardel Maracyn® is a specially compounded broad-spectrum antibiotic (erythromycin) which has been shown to be effective against a number of pathogenic bacteria associated with ornamental fish disease. It was developed for complete but gradual dissolution in water. It is primarily effective against gram-positive pathogenic organisms. Symptoms of clamped fins, swollen eyes, heavy or rapid breathing, patchy coloration, change in swimming behavior or corner-hiding can be signs of possible bacterial infection.

All suspected bacterial infections should be treated immediately.

Mardel Maracyn® is non-toxic to invertebrates. Follow dosage directions as detailed on the product packaging.

Directions & Dosage

Add the contents of one package per 10 gallons of water and repeat every 24 hours for 5 days. Repeat this 5 day treatment only once if needed.

Important: Treat for 5 days even if visible signs disappear. Maracyn® may be used concurrently with Coppersafe®, Maracyn® Two or Maracyn® Oxy. Maracyn® will not harm invertebrates. Maracyn® may cause excessive foam production in protein skimmers. Stop use of skimmer during treatment if excessive foaming is a concern.


Note: Consult a veterinarian or fish expert for assistance in diagnosis of disease causing organisms and treatment options.

Treating an aquarium less than 10 gallons? Watch for instructions!


Size Treats Item Number
8 count N/A 46000
24 count N/A 46040
60 gram jar 1,000 Gallons 46045



See below for frequently asked questions regarding Mardel Maracyn®.

  • How do you recommend using/dosing your beneficial bacteria during a 5-day Maracyn/Maracyn Two regimen?

    Using the maintenance dose should be adequate, which is 2oz per 10 gallons.

  • My box was missing the instruction insert, is it available online?

    Yes, it is listed under the resources tab of our medications. You can view and/or download there.

  • How do I treat small aquariums (betta bowls) with Maracyn®?

    What you are going to do is to break one packet of Maracyn®/Maracyn® 2 (which treats 10 gallons) into 10 equal doses (each of which will treat one gallon).

    You will need the following supplies:

    - A packet of Maracyn®/Maracyn® 2
    - Small cup - This can be a measuring cup that comes with an over the counter allergy medicine, a kitchen measuring cup (⅛ or ¼ cup sizes are fine) or even a shot glass!
    - Watertight container with lid - An empty water bottle is fine, a resealable kitchen container. The only requirement is that it must hold 10 full measures of the small cup described above.
    - Sticker or tape that can be written on
    - Pen to write on the sticker or tape


    1. Get your small cup and fill it with water (dechlorinated is best, if not tap water will be okay). Pour it into your watertight container. Repeat nine times so that you have ten measures of water from your small cup inside the watertight container.

    2. Open the packet of Maracyn®/Maracyn® 2 carefully and pour into the watertight container.

    3. Close container and shake vigorously to mix thoroughly. Do so until all Maracyn® 2 powder is dissolved. You now have ten 1-gallon doses of Maracyn® 2 mixed!

    4. Label the watertight container with sticker or tape marked, “Maracyn®/Maracyn® 2 Fish Medication.”

    5. For the first day, add two small cups of the Maracyn®/Maracyn® 2 solution to your betta bowl. Make sure you use the same small cup you used to fill the container! When done, place the watertight container with the Maracyn®/Maracyn® 2 solution in the refrigerator. Shake well before adding one cup daily for the next four days. Repeat this 5-day treatment only once if needed. This will require mixing another Maracyn®/Maracyn® 2 solution due to the double dosage on the first day.

  • Will I see an ammonia or nitrite spike after using Maracyn®?

    Almost all nitrifying bacteria are gram-negative and not adversely affected by Maracyn® and you should not see either an ammonia or nitrite spike.

  • I’ve used Maracyn®/Maracyn® Two twice now for two full treatments and it does not appear to be working, can I treat a third time?

    That would indicate that the source of the problem is not affected by the active ingredients in Maracyn®/Maracyn® Two. In this situation, rather than repeating an ineffective treatment, using a medication with a completely different active ingredient might be the best solution. So if Maracyn® was not effective, try Maracyn® Twoand vice versa.

  • Can I use Maracyn® and Maracyn® 2 simultaneously?

    Yes. Maracyn® targets gram-positive bacteria and Maracyn® 2 targets mostly gram-negative bacteria. They may be used in conjunction with one another. However, in doing so, the balance of beneficial bacteria in your aquarium, including nitrifying bacteria to remove ammonia and nitrite will be adversely impacted. Fritz strongly recommends the following: *For freshwater aquariums, have on hand and ready to use FritzZyme® 7 to replace the nitrifying bacteria that naturally remove ammonia and nitrite and FritzZyme® 360 to replace the other beneficial bacteria in the aquarium. *For saltwater aquariums, have on hand and ready to use FritzZyme® 9 to replace the nitrifying bacteria that naturally remove ammonia and nitrite and FritzZyme® 460 to replace the other beneficial bacteria in the aquarium.

  • Can I use Mardel Maracyn at same time as Prazipro?

    I have a puffer that possibly has fluke and its eyes are cloudy, I just placed him in a quarantine tank treating with Prazipro . I was wondering can I use Mardel Maracyn plus at the same time for cloudy eyes without making it toxic with both medications?

    Combining treatments will be fine.  Just be sure to monitor your ammonia levels while using any treatment.

  • Will the addition of beneficial bacteria interfere with the Maracyn?

    No, the introduction of nitrifying bacteria will not interfere with the Maracyn, but you are correct that the antiobiotics will negatively affect the nitrifiers. We get very few complaints about ammonia spikes from using these meds, but the combination of cold weather and other factors may be causing some higher than normal ammonia spikes as well. You may want to consider a chemical ammonia remover, like Fritz ACCR to help alleviate the temporary ammonia spike as well.

  • Can I use Mardel Maracyn and Pimafix simultaneously?

    Yes, they can be used together. Just always be sure to keep an eye on your ammonia when using any treatment.

  • I think my fish has a bacterial infection, how do I know which product, Maracyn® or Maracyn® Two, to use?

    If you see specific symptoms mentioned on the packaging or enclosed brochure (body fungus for Maracyn®, Fin and Tail Rot/Popeye/Dropsy for Maracyn® Two) treat accordingly. However, if the symptoms are less specific and harder to identify, start with Maracyn® Two(which is a broader spectrum product) before using Maracyn®.

  • Will Maracyn or another Fritz product work on cyanobacteria?

    Health Canada has taken Erythromycin of the pet store shelves. I have used it to treat cyanobacteria (“blue-green algae”) in the past with unfailing success. Can you please advise if Maracyn or another Fritz product will work on cyanobacteria.

    The only product we have that can be sold in Canada is our FritzZyme 360 & 460. These are biological products that will compete with cyano.  Although not a quick fix like Erythromycin, using as a regular part of maintenance will help prevent cyano breakouts.

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