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Understanding the Active Ingredients in Multi-Purpose Aquarium Medications

Sick fishIn the first two parts of this series, we discussed anti-bacterial and anti-parasitic ingredients but there are multi-purpose aquarium medications that are available to aquarists treat more than one symptom. Also, we often see more than one type of infection at a time as one can lead to another. Parasites may lead to bacterial infections at the wound site, and secondary fungal infections may occur as a result of a bacterial infection. These ingredients listed here may be effective for more than one type of disease or outbreak.


Acriflavine is used as an active ingredient to treat a number of conditions. It is an antiseptic that has been shown to be successful in treating fungal infections on fish as well as to treat some bacterial and parasitic infections. It can be used against two of the most resistant infections in the aquarium hobby: Oodinium (parasitic) and Columnaris (bacterial). Acriflavine is generally used for infections based in the slime coat and skin of the fish, not for “larger” parasites like Ich or worms.

Formaldehyde/ Formalin:

Formaldehyde is well-known as a preservative, especially for scientific specimens, but it is also used in medications and diluted solution of formaldehyde gas are found under the name Formalin. Formalin by definition is usually about 37% formaldehyde. Formaldehyde and Formaline are both used to treat fungal infections and some parasites – including the notorious Ich – but can be dangerous, especially to invertebrates (after all, parasites are invertebrates). Most formaldehyde-based medications work better as a bath or dip instead of being used to treat the entire system, and any of these medications should never be used with invertebrates that you want to keep alive. Formalin also depletes the oxygen in the water very quickly, so the water must remain well-aerated, especially when the concentration is high and water movement is low, as in a dip. Some of these medications can also be used to keep fish eggs fungus-free.

Malachite Green:

Malachite Green, also known as Victoria Green, is actually a blue dye, not green, that is used in some medications for its antiseptic properties. It is mainly used in freshwater aquariums to treat some parasites like Ich and worms and can also be used to treat fungal infections on fish eggs. It is a toxic substances that is not safe for sensitive fish or invertebrates however. Some research also shows that the toxicity increases in higher pH so medications with Malachite Green should be used with caution in saltwater aquariums. Since this active ingredient is also foremost a dye, it will dye the water temporarily during treatment – and surfaces, clothing, skin, silicone, etc. for much longer. Malachite Green is effective against parasites but its tendency to be dangerous to the organisms you are trying to save means that it should be used with caution and care.

Methylene BlueMethylene Blue:

Though it has many other uses that it is often marketed for, Methylene Blue tends to work best to prevent fungal and bacterial infections on fish eggs before they hatch. It can also be effective against parasites like Ich and protozoans – though not as effective as some of the other medications already discussed. Methylene Blue is also helpful as a dip instead of as a treatment for the entire aquarium when treating new arrivals or minor parasitic and bacterial infections. This ingredient, much like Malachite Green, will dye the water blue for a time.


Metronidazole has some antibacterial properties but it is most commonly used as an anti-parasitic medication to treat protozoan and flagellate infections. It can be used to treat Ich and Cryptocaryon infestations as well as Head-and-Lateral-Line-Erosion caused by flagellates (not to be confused with a similar condition often caused by improper nutrition). Metronidazole is also safe for many fish that are sensitive to the copper-based alternatives often available for the same conditions. This ingredient is not very water-soluble however. To get the most out of it, mix powdered forms thoroughly with water before adding it to the aquarium or mix with food and let it absorb into the food for several hours before feeding it to the fish.

Pimenta racemosa Oil (West Indian Bay Tree):

Pimenta racemosa oil is used in much the same as Tea Tree oil. The West Indian Bay Tree is a Caribbean plant whose oil has anti-fungal and anti-bacterial properties. Like melaleuca oil, it is not a very strong medication and can be used to treat mild cases of external fungus and external and internal bacteria.

Sulfathiazole Sulfathiazole:

Sulfathiazole is an antibiotic that works well to treat bacterial and fungal conditions like Septicemia, Columnaris, Fin and Tail Rot and minor damage cuased by fin nipping. It also has some effectiveness against parasitic protozoan infections, but not as much as some of the other medications already mentioned. It is safe for use with most fish since it acts on the cellular level of the bacteria to prevent the bacteria from synthesizing folic acids. Sulfathiazole medications can be used in freshwater and saltwater aquariums but tend to work best in higher pH environments. It is often used in combination with other sulfa-based active ingredients lie sulfamethazine and sulfacetamide. Use extreme caution with this medication if you or any other person maintaining the aquarium has an allergy to sulfa-based medications.

These medications and ingredients are only a sampling of those available to treat aquarium and fish conditions. Always be sure to choose carefully when treating any sick fish and follow all of the instructions for each medication you use. If you have any questions about these or any other medications or treatments, feel free to let us know in the comments or email marinebio@thatpetplace.com!

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!).