Fritz Maracyn®

  • Highly Effective Treatment for Bacterial Infections in Fish
  • May be Used Along with Coppersafe®, Maracyn® Two, or Maracyn® Oxy

  • Easy to Measure Packet Dosing System

  • Bulk Jars Available for Larger Systems

  • Safe for Fresh and Saltwater Systems

  • Effective Against Gram-Positive Pathogenic Organisms

About Fritz Maracyn®

Maracyn® is a specially compounded broad-spectrum antibiotic 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. 

Provides relief from bacterial infections such as body fungus, bacterial gill disease and popeye. Also helpful against mouth fungus, open lesions, hemorrhagic septicemia, and slime disease. Symptoms of possible bacterial infection can include clamped fins, swollen eyes, heavy or rapid breathing, patchy coloration, change in swimming behavior or corner-hiding. Consult a veterinarian or fish expert for assistance in diagnosis of disease causing organisms and treatment options.

All suspected bacterial infections should be treated immediately.

Active ingredient: Erythromycin

Directions & Dosage

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

Day 1: Add the contents of one packet per 10 gallons of water.
Day 2: Add one packet per 10 gallons of water.
Day 3: Add one packet per 10 gallons of water.
Day 4: Add one packet per 10 gallons of water.
Day 5: Add one packet per 10 gallons of water.
Day 6: Repeat this five-day treatment if symptoms improve, but have not completely disappeared. If no improvement has occurred, try our Maracyn® Two and/or Maracyn® Oxy treatments.

  • For best results, use Maracyn® in a quarantine tank
  • Remove carbon during treatment, maintain adequate water flow or aeration
  • Discontinue use of skimmer during treatment if foaming is a concern
  • UV Sterilizer can be used during treatment
  • Always complete a full course of treatment, even if symptoms improve or disappear before the fifth dose
  • No contraindications: can be used with all of our other products and meds
  • Safe for use with invertebrates
  • Some more sensitive plant species may display yellowing, but this condition generally improves after the treatment; remove any plants if concerned
  • Do not perform water changes during the course of treatment. If ammonia or nitrite is a concern, detoxify during treatment with Fritz A.C.C.R. or Fritz Complete
  • Overdosed? Perform a large water change if distress is observed

Note: FOR AQUARIUM USE ONLY. Consult a veterinarian or fish expert for assistance in diagnosis of disease causing organisms and treatment options. If you are unsure of the specific infection, Maracyn® may be used concurrently with Coppersafe®, Maracyn® Two or Maracyn® Oxy.

Treating an aquarium less than 10 gallons? Check our FAQ for some quick dilution recipes or watch our instructional video below.


Size Treats Item Number
10 count 100 gal / 378 L 46001
20 count 200 gal / 757 L 46002
100 g jar 1000 gal / 3785 L 46046
400 g jar 4700 gal / 17,791 L 46051



See below for frequently asked questions regarding Fritz Maracyn®.

  • 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.

  • 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.

  • 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.

  • Is Fritz Maracyn the same as Mardel Maracyn?

    Yes, we've changed over the name, but the formula, dosage, and quality are all the same as you may have used in the past. 

  • 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.

  • 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 use dry medications in tanks smaller than 10 gallons?

    Dividing up our powder packets accurately for use in smaller tanks can be tricky, and to get the proper dose we recommend making a concentrated solution that is much easier to measure. We've outlined the process in the video below (or linked here) to help.


    Some example recipes:

    For a 5 gallon aquarium: mix/dissolve one packet mixed in 1/2 cup of water from the aquarium. One 5 gallon dose is equal to 1/4 cup of solution.

    For a 1 gallon aquarium: mix/dissolve one packet in 2.5 cups of water from the aquarium. A 1 gallon dose is equal to 1/4 cup of solution. 

    For tanks in between the previous options, add the amount of 1 gallon doses equal the gallon size of your tank.

  • 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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