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Understanding the Active Ingredients in Antibacterial Aquarium Medications

My poor clownfish has a bacterial infection behind its right eye.Understanding what a medication contains can often be as or more important than understanding what it treats. I’ve compiled just a few of the most common active ingredients found in many of the most popular aquarium medications. This list is not all-inclusive but may hopefully help to unravel the why’s and how’s of some medications.

Remember, some of these active ingredients have more than one use and the medications they are in may be marketed for different uses. Antibacterial medications may be included in anti-parasitic medications and some anti-parasitic ingredients may also be useful in fungal infections but the uses I’ve listed are the most common or most effective in the aquarium trade. Always remember to properly diagnose conditions and diseases before medicating and follow the manufacturer’s instructions for any medications.

Part 1: Antibacterial ingredients in aquarium medications

These ingredients of common bacterial fish medications are used to treat different types of bacterial infections. Some are broad spectrum, general medications while others are geared towards Gram-negative or Gram-positive bacteria only.  The Gram designation refers to a testing system named for the scientist who developed it, Hans Christian Gram.  Known as Gram Staining, bacteria samples are treated with a purple dye under microscope, the bacteria who accept the dye, and turn purple are Gram-Positive.  Bacteria that do not accept the stain are Gram-Negative, and appear pink.  These two groups are the largest two types of infectious bacteria.  If you try a Gram-Negative medication, and it is ineffective, then you may need to switch to a Gram Positive medication. Some antibiotics may also kill off the beneficial bacteria in the aquarium, as most nitrifying bacteria are Gram-Negative, and will affect the biological stability of the system.

Amoxicillin/ Ampicillin/ Penicillin:

The “-cillins” have been well-known for decades as popular treatments for human infections, but some aquarium medications are also made with these active ingredients. All three derivatives are used to treat bacterial infections. Amoxicillin is the most broad-spectrum of the three and treats Gram-positive and Gram-negative infections. Ampicillin treats some Gram-negative infections but is most effective on Gram-positive bacteria. Penicillin is used for Gram-positive bacteria. Aquarists who are allergic to Penicillin or any other -cillin derivatives should use extreme caution if using these medications in their aquarium.


Erythromycin is a antibiotic very similar to Penicillin. It is more effective in freshwater aquariums than in saltwater but can be used to treat difficult eye problems. Some aquarists also recommend Erythromycin to treat cyanobacteria blooms but this should be used with caution and the cause of the cyanobacteria still needs to be addressed. While Erythromycin is useful with especially gram-positive bacterial infections, it can also kill the usefully nitrifying bacteria in the tank and may cause some instability while the helpful bacteria population recovers. Avoid using this medication if the bacteria population is already unstable, like in new or newly established aquariums.

Furazolidone/ Nitrofurazone:

Furazolidone and Nitrofurazone are closely related anti-bacterial medications that are often used together in medications. These medications can be used in freshwater and saltwater and are often used with pond fish to treat Aeromonas infections. It is also often recommended to treat Columnaris, Vibrio, and Furunculus. While it can be helpful against marine skin infections, it usually works better in lower pH environments. Nitrofurazone is usually found in yellow crystal-form and can temporarily dye the water yellow.

Kanamycin sulfate:

Kanamycin sulfate is another antibacterial medication but unlike Furazolidone and Nitrofurazone, Kanamycin is most effective in higher pH and is therefore more effective in saltwater than in freshwater, although it can be used in either. Kanamycin targets mostly gram-negative bacteria but will also work on some gram-positive bacterial infections. It is considered safer for the biological filtration in the tank than medications like Erythromycin. Kanamycin is recommended for conditions like dropsy, pop-eye, columnaris, and septicemia.

Melaleuca in Aquarium MedicationsMelaleuca Oil:

Melaleuca oil comes from the Tea Tree, Melaleuca alterniflora, an Australian native similar to the Myrtle trees in the United States. The oil from this tree is antibiotic and is used in not only aquarium medications but also in human and animal antiseptics to help kill bacteria on wounds and dry up some skin conditions (you can usually buy Tea Tree oil in the first aid section of your pharmacy to treat poison ivy and similar conditions). Aquatic medications use melaleuca oil as an antibacterial medication. It is not as strong as some of the other antibiotics and antibacterials that have already been mentioned, but it is gentle and effective against mild problems like frayed fins due to bullying.


Neomycin sulfate is very popular in home first aid kits as an active ingredient in the antibiotic Neosporin®. It is also used in aquarium medications as a broad spectrum antibiotic used to treat bacterial infections and wounds. Some research has also shown that Neomycin may have some impact on some of the notoriously resistant bacterial infections like Aeromonas, Pseudomonas and Mycobacteria. Neomycin can be found in powders forms, in liquid medications, and in some topical solutions much like the Neosporin® used on our own cuts and scrapes.


Much like Sulfathiazole, Tetracycline is a bacteriostatis antibiotic – it prevents bacteria from producing the proteins it needs to multiply. Tetracycline can be used to treat a wide range of bacterial infections, both gram negative and gram positive, but is only for use in freshwater aquariums.

Keep an eye out for next week’s continuation where I’ll be discussing common anti-parasitic medications and their active ingredients. Let us know in the comments if you’ve had any particular successes with any of the medications listed above!


  1. avatar

    Can I use fish Antibacterial solutions for adding to agar plates to cut down on contams? It can’t be antifungal as I grow mushrooms started on agar and it’s a fungus of course… is there a med for my agar plates?

  2. avatar

    Hi Lawrence, We don’t have any information on antibacterial medications for agar plates. I would recommend contacting a business or organization similar to what you are doing with the agar plates and mushrooms for recommendations, especially if you are growing them for human consumption. Medications like those used in aquariums are generally not safe for or for things grown as food for humans. I did a quick Google search for “mushroom agar plates” that came up with lots of results that may help you get in touch with someone with similar experience to yours.

  3. avatar

    How to remove the Maconkey Agar in aquarium that is a pathogen for my goldfish if it appears. Changing water, scubing tank just remove it but it will be back when the aquarium has some waste by foods, fish poops.. it makes water cloudy, sticking at the tank and stuck inside the filter but wipe the tank wall by the white napskin could see the pink color on there. Could smell it through the top of tank, smells sour like the yourt.
    My tap water is softener, has very low Ph. i’m using Seachem Gold Buffer non-phosphate to buff Ph up. Changing brand new tank and brand new equiments, using chlorine disinfected and run the Nitrogen cycle again, using inline UV sterilizer and media reactor phosphate removal after the cycled done but it dosen’t work.. It keep coming back..

    I have 3 goldfish beeing dropsy when Maconkey Agar appears. Where is that come from ?? ( tap water, fish, or in the house atmosphere..). How could find out what’s cause this happen?How to remove/treat or control it for it not coming back and keep aquarium healthy?

    This is a negative -gram, how to treat if doesn’t kill the nitrifying bacteria.. ??
    It’s challenge me, turning my fish keeping life more difficult.

  4. avatar

    Hello Chau, Agar is not bacteria. It is used in labs as a media for culturing some bacteria but it is a kind of jelly made from algae. It sounds like your tank is having water quality issues and may not have sufficient filtration but without knowing more about it (tank size, type of filtration, size and number of fish, parameters like pH, Ammonia, Nitrite, Nitrate just to start with), it is difficult to say exactly what the problem is. If you would like to send us an email with more details about the tank, we can help figure out how to get the tank healthy. You can email us at fish@thatpetplace.com with full details about the tank.

  5. avatar

    Very appreciated to your reply. I did sent the email for more information about the tank and the problems . Hope you have the reply back to me soon. Thank you very much

  6. avatar

    Hello Chau, I responded to your email. If you have any other questions or concerns, feel free to let us know. You can also contact the Fish Room staff at 717-299-5691 to discuss your issue with one of our staff.

  7. avatar

    Just found out that Tetracycline is very similar to Doxycline Hyclate (found in API Fin and Body Cure). Both are antibiotics for wide varieties of bacterial infection. So, if you don’t know exactly what you fish has, using DH is a good idea. My black molly boy had white thing growing on his mouth and fin 2 days ago. Used Nitrofurazone/Furazolidone (in Jungle Fever) as I thought it was fungus but it had actually spread to his body. So, it couldn’t be fungus and today, I switched to DH and in evening, saw improvements already as the whiteness looks noticeably faint.

  8. avatar

    I’m currently using doxycycline to treat fin rot and a swim bladder issue in my Betta. He’s in a 10gal with a few trumpet snails. Will all the nitrifying bacteria die during this treatment?

  9. avatar

    Hello Natalie, Doxycycline is safer than some other medications but any antibiotics like these can harm the beneficial bacteria.

  10. avatar

    I love your blog and have read the article about different fish medications. I’ve been adding the recommended amount of aquarium salt, erythromycin, and pimafix (all API brand). I change the water weekly and clean the tank thoroughly. Yet for about a month-2 months, my fish looks like the photo attached. Colorless, white patches on tail, not eating, troubke swimming, and lethargic.
    Any additional ideas about how I can help him would be very appreciated. Thanks! 🙂

  11. avatar

    Hi Kayce, There is no photo attached here. You can email fish@thatpetplace.com with information about your tank if you would like someone here to take a look at it for you. Some important answers we would need: Did you remove any carbon or other chemical filtration from the tank while you were treating? What kind of fish do you have in the tank and how large is the tank? What kind of filtration is on the tank? When you say you “change the water weekly and clean the tank thoroughly”, what does that mean? What percentage of the water do you change each time? Have you tested the water quality and what are the values for pH, Ammonia, Nitrite, Nitrate and temperature to start with?

  12. avatar

    You said, “While Erythromycin is useful with especially gram-positive bacterial infections, it can also kill the usefully nitrifying bacteria in the tank and may cause some instability…”

    MANY other sites have said it is safe to use with nitrifying bacteria, won’t harm plants, won’t harm fish or invertebrates.

    I am confused why you caution its use.

    PS…instructions say 1 packet per 10 gallons, but is not telling me what % or strength is. For example, I have access to 250 mg tablets used for human bacterial infections. A pharmacist said it is the same drug as used in pet trade. How much of a 250 gal tablet would I add per 10 gal?

  13. avatar

    Hi Norm, Erythromycin is far safer than other medications since it mainly targets gram-positive bacteria and nitrifying bacteria is gram-negative but any antiobiotics and anti-bacterial medications have some risk for the bacterial colonies. As long as it is dosed safely, it usually doesn’t cause issues directly. Water quality can be affected as it kills off other bacteria in the tank. We generally don’t recommend using medication that wasn’t formulated for aquarium use since it may contain binders and other additives that can affect that water quality but most commercial aquarium medications dose at about 20mg/gallon repeated over the course of four says with 25% changes in between.

  14. avatar

    My established freshwater tank is no longer cycling nitrites and I’ve been searching for the answer since. Thank you so much for this article as I was using EM.

  15. avatar

    I have or had a community fish tank containing many different species from Algue eaters , clown loaches , eels , guramis , plus many more , the tropical fish tank capacity is 300 litres and incorporates 2 x canister filters , non carbon is present , I noticed some fin rot on some of my tiger barbs , then I was recommended then sold tri – sulphate to treat the problem , I followed the directions and added the product to the fish tank
    The following day my clown loaches were dead then over the presiding week the community tank . Would the tri sulphate have killed off the beneficial bacteria ?

  16. avatar

    Hi Mark, I’m not familiar with any medication known as “tri sulphate”. Do you mean API’s Triple Sulfa? That medication is typically safe for biological filtration and I wouldn’t expect it to kill off the beneficial bacteria. Without much more information (exact medication used, the dosage, the water chemistry before and after treatment, the symptoms you were seeing, etc), I really wouldn’t be able to hazard a guess as to what might have happened. Feel free to give our staff a call at 717-299-5691 if you would like to discuss the situation with one of our associates.

  17. avatar

    Hello, I’m currently treating some german blue rams with maracyn (erythromycin) for popeye Im on day 3 and it doesn’t seem to be getting better. They seem more stressed. I’m wanting to switch to fin and body cure (doxycycline) should I finish the erythromycin through day 5, or can I use both at the same time?

  18. avatar

    Hi Julie, Medications should always be used as directed for the length directed unless the fish are showing a severe reaction to it. In this case, the medication should be removed using carbon or another chemical-removing material before using another medication.

  19. avatar
    Gloria Sanchez

    Hi, my black phantom tetra has two little white cotton balls on each side of its tail and a tiny bit of white coming out of each Gill, I have treated him with ParaGard for one week, waited a week and now trading with China Plex, nothing seems to be working, I believe it’s a fungus? Which would be Gram negative? My water parameters are perfect and I clean half the gravel and a partial 15% water changes weekly in my 32 gallon aquarium, I have a total of five black phantom catchers and six red eye touches, three or four of them also have frayed fans I believe from fighting, Please advise, thank you so much

  20. avatar

    Hi Gloria, If you can email us photos or video of what you are seeing to fish@thatpetplace.com along with information about your tank (size, residents, filtration, the water parameters levels for at least Ammonia, Nitrite, Nitrate and pH) and the details of any treatments you’ve done, we can take a look at it and help you figure out the best treatment.

  21. avatar

    Hi, I have a 120lit. tank with 5 goldfish, 5gupies and 10 neon tetras.i I have 3 filters enough for 1000 liters, and one carbon cube . When I added the guppies my goldfish developed finrot. They remain active and hungry. For the last month, first treatment was myxazin and second was melafix both as instructed. I think it’s time to try erythromycin since they are getting worst. Water conditions are checked every other day and are normal. I vacuum the gravel every week (sometimes twice a week)and every other week change 25%of water making sure to keep the temperature stable. No pet store in Greece has the API erythromycin and also there are no fish-vets. My pharmacist can give me pure erythromycin or any other needed antibiotic as a pure form (not already made for human use for example tablet or liquid) , but asked me for exact concentration in order to fix me the individual doses. Please advise me how to proceed, I am desperate to save them.

  22. avatar

    Hi Nantia, I would need some more information to help you diagnose them properly. Keeping tropical fish like tetras and guppies with coldwater fish like goldfish can definitely be tricky and 120 liters is very small fish so many fish (especially high waste producers like goldfish). What temperature do you have the tank at now? Have you tested the water parameters and what are the values for pH, Ammonia, Nitrite and Nitrate to start with? You mentioned that you siphon the gravel weekly or twice a week in addition to biweekly 25% water changes and that seems like quite a lot. How much water are you changing when you siphon your gravel in addition to the biweekly water changes? It is possible that you may be removing too much water and therefore too much of the beneficial bacteria that helps to control Ammonia and Nitrite levels. Also, you mention a “carbon cube”…did you remove this while medicating? Carbon removes medications and, in addition to the water changes especially, would have removed any medications you were treating with. Feel free to email us at fish@thatpetplace.com with this information and photos of what you are seeing if you’d like so we can help you with your tank in more detail than I can here.

About Eileen Daub

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Marine Biologist/Aquatic Husbandry Manager I was one of those kids who said "I want to be a marine biologist when I grow up!"....except then I actually became one. After a brief time at the United States Coast Guard Academy, I graduated from Coastal Carolina University in Myrtle Beach, South Carolina in 2004. Since then, I've been a marine biologist at That Fish Place - That Pet Place, along with a Fish Room supervisor, copywriter, livestock inventory controller, livestock mail-order supervisor and other duties here and there. I also spent eight seasons as a professional actress with the Pennsylvania Renaissance Faire and in other local roles. If that isn't bad enough, I'm a proud Crazy Hockey Fan (go Flyers and go Hershey Bears!).