Your fish has ragged fins, a white patch, or red streaks, and you're staring at a shelf of names that all sound alike: Fish Mox, Fish Dox, Fish Zole, Fish Cipro. They are not interchangeable. Each one belongs to a different drug family, and each family works on a different kind of problem. Pick the wrong one and you lose days while the infection spreads. This guide explains what each medication actually is, when it makes sense, and the steps that matter more than any pill.
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Often it isn't, at least not at first. In its 2025 update of Use of Antibiotics in Ornamental Fish Aquaculture, University of Florida IFAS Extension notes that bacterial infections are often secondary to problems like poor water quality, sudden temperature changes, poor nutrition, or rough handling. The bacteria are usually already in the tank. Stress lets them take over.
So the first tool isn't a capsule. It's a water test. Check ammonia, nitrite, pH, and temperature, and look closely for parasites. If you skip this, the medication may clear the symptoms for a week while the real cause keeps going. For a walkthrough of testing and the most common diseases, see our Aquarium & Fish Health 101 guide.
Here's the part most medication guides skip: UF/IFAS says antibiotics don't cure a fish by themselves. They hold bacteria back long enough for the fish's own immune system to finish the job. A stressed fish in bad water can't do that, whatever you put in the tank.
The quickest way to choose is by drug family. University of Florida extension specialists group aquarium antibiotics by the kind of bacteria they hit hardest, and the difference matters: most fish pathogens, including Aeromonas, Vibrio, Pseudomonas, and the columnaris bacterium, are gram-negative. The chart below matches each product we carry to its family.
| Product | Drug family | Works best against | Watch out for |
|---|---|---|---|
| Fish Mox, Fish Pen, Fin Pen | Penicillins (amoxicillin, penicillin) | Mainly gram-positive bacteria, such as Streptococcus | Best when the infection is gram-positive; pair with a broad-spectrum option |
| Fish Dox | Tetracyclines (doxycycline) | Broad-spectrum; the tetracycline family still works on most columnaris cases | Hard water and light weaken tetracyclines in the tank |
| Fish Ceph | Cephalosporins (cephalexin) | Mostly gram-positive, some gram-negative | An older member of the family; not a cure-all |
| Fish Sulfa 960 | Potentiated sulfa | Broad-spectrum; potentiated sulfas still work where plain sulfas fail | Plain sulfas lost ground to resistance |
| Fish Zole | Nitroimidazole (metronidazole) | Internal flagellates like Hexamita, anaerobic bacteria | Treats a parasite, not most skin infections |
| Fish Cipro | Fluoroquinolone (ciprofloxacin) | Wide range, including resistant bacteria | Keep it as the last resort |
Drug family notes summarized from UF/IFAS Extension FA084. Always follow the dosing on your product's label.
Fish Mox contains amoxicillin, and Fish Pen and Fin Pen contain penicillin. UF/IFAS lists the penicillins as most effective against gram-positive bacteria such as Streptococcus. Because most fish infections are gram-negative, the same guide suggests a broad-spectrum drug as the usual first choice when you don't know the exact bacteria.
That doesn't make Fish Mox a bad buy. It's one of the most trusted names in fishkeeping for a reason: it's affordable, easy to dose, and a strong match for Streptococcus and other gram-positive infections. Many experienced keepers keep it on the shelf next to a broad-spectrum option, so they're ready for either type of problem.
A trusted, affordable amoxicillin option for infections like Streptococcus. Great for smaller tanks.
$20.99 Shop Now →
Fish Mox 500mg Capsules, 30 Count
The fishkeeper favorite in a higher strength for larger tanks.
$28.79 Shop Now →
Fin Pen 250mg Tablets, 30 Count
Penicillin in tablet form, aimed at gram-positive bacteria such as Streptococcus.
$23.99 Shop Now →
Fish Pen 500mg Tablets, 30 Count
Higher-strength penicillin tablets for bigger volumes of water.
$28.79 Shop Now →
Fish Dox contains doxycycline, part of the tetracycline family. UF/IFAS describes oxytetracycline and related antibiotics as broad-spectrum, and notes that oxytetracycline still works adequately against most cases of columnaris disease. That breadth is why Fish Dox is our best-selling fish medication.
Two catches. Calcium and magnesium bind to tetracyclines and make them less active, so hard water weakens a bath treatment. Tetracyclines are also light-sensitive and turn brown as they break down, which fouls the water. Dose in a dim spot and change the water when the treatment period ends.
Fish Dox 100mg Capsules, 30 Count
Doxycycline, a broad-spectrum tetracycline. Our best-selling fish medication.
$43.99 Shop Now →
Fish Sulfa 960 is a potentiated sulfa, and it's the stronger fit for gram-negative problems like fin rot, ulcers, open sores, and red streaking. UF/IFAS notes that plain sulfa drugs lost effectiveness through overuse, while potentiated sulfas are still effective.
Fish Ceph contains cephalexin, an older, first-generation cephalosporin. UF/IFAS says the cephalosporin family covers some gram-positive and many gram-negative bacteria, but its examples are newer injectable drugs. Cephalexin itself tends to be strongest on gram-positive bacteria, so treat it as a middle option rather than a cure-all. The 60-count pack is the better value for larger tanks.
Fish Sulfa 960mg Tablets, 30 Count
A potentiated sulfa tablet often chosen for fin rot, ulcers, and red streaking.
$32.95 Shop Now →
Fish Ceph 250mg Capsules, 30 Count
Cephalexin, a first-generation cephalosporin for ornamental fish.
$20.49 Shop Now →
Fish Ceph 500mg Capsules, 60 Count
The larger 60-count pack for big tanks or repeat courses.
$38.49 Shop Now →
Mostly parasites. Fish Zole contains metronidazole, which targets internal flagellates such as Hexamita plus anaerobic bacteria. A veterinarian writing for The Vet Desk (Dr. Samantha Devine, DVM, 2025) says metronidazole is the treatment of choice for Hexamita, ideally mixed into food rather than added to the water.
The same vet notes that many hole-in-the-head cases show no Hexamita at all. Stress, poor filtration, and a low-fiber diet are common drivers. So use Fish Zole when the signs fit, such as pitting on the head of a cichlid or white stringy waste, and fix water and diet at the same time.
Fish Zole 250mg Tablets, 30 Count
Metronidazole for internal flagellates like Hexamita and anaerobic bacteria.
$33.99 Shop Now →
Fish Cipro contains ciprofloxacin, a fluoroquinolone. UF/IFAS says the FDA classifies fluoroquinolones as being of high regulatory concern, and that aquarium keepers use them as a higher-powered drug of last resort when bacteria resist other treatments.
That's the right way to think about it. Using your strongest option first leaves you nothing when an infection returns. Reach for Fish Cipro only after a broad-spectrum course has failed, and ideally with advice from an aquatic vet.
Fish Cipro 500mg Tablets, 30 Count
Ciprofloxacin, a fluoroquinolone kept for infections that resist other treatments.
$46.99 Shop Now →
Treat in a separate hospital tank. UF/IFAS warns that antibiotics added to the water can kill the nitrifying bacteria in your biological filter, and recommends a separate container for bath treatments. A small bare tank with a heater, a simple filter, and its own net is enough.
From the same extension guide, the rules that matter most:
And one safety rule for the household: these products are for ornamental fish only. A 2020 PLOS ONE study found 24 online vendors selling fish antibiotics, and 2.4% of 2,288 customer reviews suggested people were taking them. Don't. They aren't approved for humans, and the dose and purity aren't checked for people.
Mini Flat Heater for Small Aquariums
Holds a small hospital tank at a steady temperature during treatment.
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16 oz Aquarium Water Conditioner
Treats tap water for the frequent water changes a hospital tank needs.
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Stress Zyme Bacteria Supplement
Helps rebuild beneficial bacteria once treatment ends and the filter goes back online.
$16.68 Shop Now →
Aquarium Power Filter with 3 Cartridges
A cheap, simple filter for a small quarantine tank.
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Healthy fish start with stable water, and stable water starts with the right tank and filtration. Whether you're adding a quarantine tank or upgrading your display, here are picks from entry level to showpiece.
Glass 29 Gallon Aquarium Starter Kit
More water volume means steadier water quality for larger fish.
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20 Gallon Aquarium Starter Kit with LED Lighting
A complete second tank that can serve as quarantine or a grow-out tank.
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Power Aquarium Filter, 3-Step, 30 to 60 Gallons
Strong filtration for the main tank, the first defense against bacterial outbreaks.
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38/55 Gallon Wood Aquarium Stand
A sturdy wood stand for a mid-size display tank.
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120 Gallon Curved Panoramic Bowfront Aquarium with Stand
A reef-ready showpiece with a gloss black stand.
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140 Gallon Seamless Glass Bowfront Aquarium with Cabinet and Canopy
Real wood cabinet and canopy around a seamless bowfront tank.
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Fin rot is usually caused by gram-negative bacteria. University of Florida extension guidance says penicillins like amoxicillin work best on gram-positive bacteria, so a broad-spectrum option such as doxycycline (Fish Dox) or a potentiated sulfa (Fish Sulfa) is often the better first pick. Fix water quality first, since fin rot often follows stress.
University of Florida extension guidance says combining antibiotics is generally not recommended, because they can interfere with each other. Most bacterial infections respond to one well-chosen antibiotic given for the full course. Switching quickly between drugs without a diagnosis is how resistant bacteria develop.
They can. Antibiotics added to the water may kill the nitrifying bacteria in your biological filter. That's why extension specialists recommend treating sick fish in a separate hospital tank, then rebuilding beneficial bacteria with a supplement once treatment ends.
Fish Zole contains metronidazole, which targets internal flagellate parasites such as Hexamita and anaerobic bacteria. It's commonly used when hole-in-the-head disease is linked to Hexamita. A veterinarian writing for The Vet Desk notes that water quality and diet should be corrected first, since many cases have no Hexamita present.
No. Fish antibiotics are not approved for people and carry no guarantee of purity or correct dose for human use. A 2020 PLOS ONE study of 2,288 online reviews found 2.4% suggested human use. See a doctor for any human infection. These products are for ornamental fish only.
Sources: University of Florida IFAS Extension, Use of Antibiotics in Ornamental Fish Aquaculture (CIR 84/FA084), updated August 2025, retrieved 2026-10-05. Online availability of fish antibiotics and documented intent for self-medication, PLOS ONE, 2020, retrieved 2026-10-05. Dr. Samantha Devine, DVM, Hole-in-the-Head Disease in Fish, The Vet Desk, October 2025, retrieved 2026-10-05. This article is general information, not veterinary advice. For valuable or very sick fish, contact an aquatic veterinarian.
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