Intermediate Topic Keeping Your Marine Fish Healthy, Part 3: The Reef Aquarium Medicine Cabinet Continued

As I mentioned in Part 1 and Part 2, we have in our forum, an excellent area for the discussion of fish disease, treatment, and diagnosis. Our venerable Humblefish even created a “sticky” thread (one that stays permanently on top) called, “Medications to Keep on Hand.” So, if you don’t know what ailment you’re dealing with, then by all means, ask questions there. You will find several very helpful stickies and plenty of forum members to answer questions.

Before I get back to my list of medicines that I think you might want to have around for your fish, let’s look quickly at some of the most common illnesses that you may run into with a reef aquarium:

A. Poisoning

B. Marine Ich (Cryptocaryon irritans, a protozoa)

C. Marine Velvet (Amyloodinium ocellatum, a dinoflagelate)

D. Brooklynellosis (Brooklynella hostiles, a protozoa)

E. Vibriosis (bacteria)

F. Viruses like lymphocystis (cauliflower disease)

G. “Pop-eye” (trauma or bacterial)

H. Worms or other parasites

I. HLLE (Head and Lateral Line Erosion)

J. Fungi

K. Assorted other nasties that I’ve forgotten

With this list of maladies in mind, let me recap. I’ve already mentioned copper (medication), methylene blue, and clove oil (eugenol).

Poisoning could be from bad water parameters or cyanide or you dosed too much of a medicine. If it’s cyanide, methylene blue (already covered) may help. If your water isn’t good quality, the way to fix that is to improve it quickly or move the fish into a better water quality area quickly. If the water quality problem is an ammonia problem, then the quick (temporary) fix is my #4.

4. Seachem Prime or equivalent: Prime can bring down ammonia levels immediately in a crisis situation. Water changes would help too, but they take longer. Better biological filtration would help, but that takes even longer. Prime brings down nitrite, nitrate, chlorine and chloramine levels, but if you’re using an Reverse Osmosis/Deionization (RO/DI) unit, you shouldn’t have a chlorine or chloramine problem to start with.

My only concern about Prime is that we don’t know what the ingredients are. Some people will not use anything in their tank if they don’t know the active ingredients. More on this later. If we don’t know exactly what Prime is, then we can’t know exactly how it works or what you’re adding to your tank. However, desperate times require desperate measures. Here is the Seachem Prime Material Safety Data Sheet (MSDS), laughable in its lack of information.

Prime (or equivalent) is not really a medicine, but since it would be used to treat medical problems, and perhaps life-threatening ones, I think it belongs on this list.

So, now we’ve covered poisoning, sedation, and euthanasia. And unfortunately there aren’t any anti-viral drugs for marine fish—at least ones that are available in the aquarium trade. So, that’s a dead end. If your fish gets a virus, the fish will live or die, but no medications will help. There is research being done on marine fish viral response and immunity.

5. Praziquantel and/or metronidazole: Praziquantel is used for worms and flukes in fish. (It’s also used in humans and dogs for various delightful parasitic worm infections.) It seems to be very well tolerated by marine fish, and many reef aquarists use it routinely in quarantine for every fish coming in. "Prazi" is also reasonably “reef safe,” which means you could use it in your display tank, although a few feather dusters might suffer. Metronidazole can usually take care of nasties that Prazi is not effective against, and metronidazole can also treat Uronema, Brooklynellosis, HLLE, and it has some anti-bacterial activity against anaerobic bacteria. Metronidazole also has some anti-inflammatory properties in humans and dogs, but I’m not sure about fish.


I must admit to a personal bias for metronidazole. I think it’s a miracle drug. I’ve personally taken it by mouth, by IV, and I have a standing prescription for it for one of my dogs. Since I always have it around the house, I probably wouldn’t buy it. And it does have its downsides with marine fish. Drug resistance can develop. Metronidazole is not always well-tolerated and can cause kidney failure in fish. And it’s not terribly water soluble.

Regarding pop-eye (letter H. above), if it’s just one eye, it’s probably trauma. The treatment is do nothing and perhaps use Epsom salts in a hospital tank for swelling or possibly a lowered salinity environment. If it’s both eyes, then it’s probably bacterial, and you would want a broad-spectrum antibiotic. We’ll look at antibiotics next.

There are several challenges for the reefer trying to treat bacterial infections in their fish. The first is that we don’t typically culture bacteria for identification and drug sensitivities for our fish like doctors do for us or vets do for our companion animals. It might be possible to do so if you have a close relationship with a veterinarian, but a fish may succumb to its illness before the cultures come back (3-4 days). So, giving a fish an antibiotic is a shot in the dark. You may suspect an infection is aerobic or anaerobic, but that’s as far as we can go. With that in mind, you might want to keep on hand one or two broad-spectrum antibiotics, knowing that fish diseases have drug-resistant versions just like our own diseases.

Culturing bacteria.

Photo is a royalty-free image from Pixabay.

Both gram-positive and gram-negative bacteria can be either aerobic or anaerobic. Gram-positive and gram-negative refers to the bacterium’s morphology and “differential staining properties” and has nothing to do with whether a bacterium is aerobic or anaerobic.

6. Antibiotics: There are tons of antibiotics available, more than I can list here. But you might want to choose one or two “broad-spectrum” ones. Maracyn Two is a possibility or Kanamycin or one of the sulfa drugs or even Furan-2 or Minocycline, which also has some anti-inflammatory properties. In you live in a small town, or your LFS is not well-stocked, you might want one or two of these drugs on hand, but it doesn’t make sense to buy all of them in advance of needing one. Interestingly enough, there are a number of fish vaccines available for bacterial infections, but I don’t know how easy it would be to get your hands on them.

7. Formalin: Formalin, a formaldehyde solution, can treat a lot of ailments. But there are a lot of buts. It’s a known carcinogen. It will kill anything—the dose makes the poison. That’s why it’s also used as a disinfectant and biocide. It reduces the oxygen in the water. Basically, you need to be really careful working with it to protect yourself and not kill your fish while you’re ridding the fish of microorganisms that shouldn’t be there.

8. Coral Dip: A coral dip is #8 not because it’s not important but because I wanted to group together the medicines for fish. Giving your coral at least a dip or bath before going into your display tank is important. Plenty of bad things can be introduced to your DT on frags. Definitely include this in your medicine chest.


There are tons of drugs/chemicals that I haven’t mentioned: chloroquine phosphate (you need a prescription from a veterinarian), Flatworm Exit (I’d like to discuss this separately, and it’s not mission-critical if you have a bunch of ugly flatworms), and dexamethasone, for example, (you need a prescription for it, and you have to be willing and able to give your fish an injection).

I also did not include any drugs for the treatment of inverts—because I couldn’t find any. I found plenty of medications derived from inverts, but I guess treating sick snails or shrimp is not mainstream.

Although methylene blue can handle minor fungal problems and protect eggs from fungi, a big or systemic fungal infection would require something stronger. While major fungal infections are common in aquaculture facilities, they are not the most common in the reef aquarium, so I have omitted these drugs here (like Acriflavine or Malachite Green) in my list of eight. Malachite green is ubiquitous in the marine aquarium trade, but it’s very toxic, stains everything blue like nobody’s business, and is hard to dose correctly.

But I think this list of eight (8) products/medicines/chemicals is a reasonably good Reef Medicine Cabinet. You might eventually end up buying more, or replacing medicines that become stale-dated, but you can handle most medical problems with these products listed. My list is 1) a copper medicine, 2) methylene blue, 3) eugenol or clove oil, 4) Seachem Prime or equivalent, 5) Prazi and/or metronidazole, 6) one or two broad-spectrum antibiotics, 7) formalin, and 8) a coral dip. Your list might be different. And make sure to have some carbon on hand, which could be necessary to absorb or lower the dose of medicine.

It’s possible that some drugs might be available and cheaper to buy as the human version or the companion-animal version than from an LFS. And if you have a pill crusher or cutter, you can make do. For the purposes of this article, I did not compare prices.

If you have a veterinarian, it’s worth discussing some of these topics with him or her. A curious and compassionate vet will help you even if he or she is not an expert on fish medicine and does not handle exotics. They have access to this information if they want to get it. You could also try to establish a relationship with a local vet if you don’t have one already.

Please be careful when you are treating sick fish. Many aquarists stick their hands in their tanks all the time and live to tell the tale. But you’re not doing your livestock any favors. Your reef is not the ocean. It’s a closed system. Residues of soap, lotion, perfume, dirt, and even the oils on your skin will bugger up your water quality.

Various medications.

This photo is a royalty-free image from Pexels.

When you’re dosing medicine or making stock solutions, try to keep your hands out of it, and protect yourself. Aside from urchins that can stick you, eels that can bite you, anemones that can sting you, zoanthids that can blind you, and all kinds of other mishaps, plenty of these medical treatments described here are toxic—that’s why and how they kill the microorganisms that they’re supposed to.

If the aforementioned weren’t enough, Vibriosis can be hard to treat, it’s caused by several different Vibrio species, and it’s also a disease that can be transmitted to humans. It’s possible that you could be allergic or (very) sensitive to some of these meds or chemicals. Some chemicals like formalin and malachite green are dangerous to handle. Chloramphenicol, an antibiotic also used in humans and dogs—often as a last resort for infections not responding to other meds—can be used in marine fish bacterial infections, but you just about need a biohazard suit to get near it.

I’m just kindly suggesting to exercise care and caution. And remember that there’s a US liquid gallon, a US dry gallon, and an Imperial gallon. Try using metric measurements for dosing. You’re less likely to make mistakes.

In humans and many animals, the relationship between stress and illness is complex but well documented. Modern medicine is wonderful, but if the same relationship between stress and illness exists for fish (and why not?), then your best chance to keep your livestock healthy is to try to keep them as stress-free as possible. That means try to keep your water pristine, don’t overcrowd your reef, choose compatible tank mates, feed your livestock appropriate and nutritious foods, give them a physical environment that at least resembles what they’re used to, and quarantine them.

Often people think of quarantine as mean or cruel to the livestock. But it’s not. It offers them a quiet, dimly lit, safe place to rest, often after a torturous and painful journey from the ocean. Think of quarantine as a vacation for your livestock.

NB: In Part 2, l mentioned that I couldn’t think of a reason why the reef aquarist would need to sedate a fish. Well, I’ve uncovered a couple of reasons while researching this article. One would be if you wanted to give a fish an injection like of an antibiotic. If you were going to do that, you’d probably need help from a vet to prepare a sterile syringe with the correct (tiny) amount needed—so, I’d probably also rely on the vet to help sedate the fish with lidocaine. The second possibility would be if you wanted to treat a prolapsed rectum with a purse stitch. I’d sedate (and again ask for the vet’s help.)

Special thanks are due today to Dr. Faye Briggs, DVM, and Dr. David MacDonald, DVM, of the Comox Valley Animal Hospital for discussing many of the topics above with me. They are both willing to help and make house calls for a sick fish. And I also wish to thank again, Reef2Reef forum user, hdsoftail1065, who is my chief researcher stateside.

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Author Profile: Cynthia White

Cynthia received her BA in English from NYU during the Paleozoic Era. She has been a freelance writer and editor for over 20 years. She has written for newspapers and magazines, both in print and online and was formerly a marketing manager for a small oil company. Her portfolio can be found here. Now she is a writer and editor on staff at R2R, where her forum nickname is Seawitch. Her build thread can be found here.

For 15 years, she kept a dozen freshwater tanks, bred cichlids--Cyphotilapia frontosa--and sold them to pet stores in Calgary. Finally, after years of study, she has come to saltwater side. She lives in British Columbia, Canada, with her husband and three special-needs dogs, a five-minute walk from the Georgia Strait.