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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.

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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!).