Aquarium fish antibiotics
Fish Zole: Metronidazole for Fish (250 & 500 mg Tablets)
An antiprotozoal for internal parasites in ornamental fish. Intended for aquarium fish only, not for human use.
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Delivery in 10–14+ business days. Restock your aquarium’s medicine shelf before your fish need it. This is for planning ahead, not same-day treatment.
No public checkout. After you place an order, our pharmacy confirms your address and sends a secure payment link. A qualified team member reviews every order for aquarium use before it ships. Your card isn’t charged until you approve the link.
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Quick answer: Fish zole is metronidazole for fish. It is an antiprotozoal and anaerobic antibiotic used mainly for internal parasites, Hexamita and Spironucleus, behind hole-in-the-head disease and Malawi bloat, especially in cichlids and discus. It isn’t a general-purpose antibiotic. A fish vet should confirm the cause; drug choice and dose are the vet’s call. For aquarium fish only; not for human use.
Not sure of your tank size? Use our free aquarium tank volume calculator to get the true water volume for accurate dosing.
What fish zole is
Fish zole is the common name for metronidazole sold for aquarium fish. Metronidazole is a nitroimidazole, both an antiprotozoal and an antibiotic against anaerobic bacteria. It works by disrupting the DNA of protozoan parasites and oxygen-avoiding bacteria. This product is the same molecule, manufactured and labeled specifically for ornamental fish.
We are a licensed, vet-staffed pharmacy, not a repackager. Our job is to sell you the right treatment, and to help you use metronidazole for the right reason. A vet confirms the cause and the dose. That care is the whole point.
| Active ingredient | Metronidazole |
|---|---|
| Strength | 250 mg or 500 mg (Forte) per tablet |
| Count | 60 tablets |
| Drug class | Nitroimidazole (antiprotozoal / anaerobic antibiotic) |
| Form | Capsules sealed in blister packs, open for tank dosing |
| Intended use | Ornamental / aquarium fish only |
| Availability | Over-the-counter for aquarium use only, not for human consumption |
| Delivery to | Free to all 50 US states & Canada, other countries, contact us |
Why blister packs, not a bottle
Each capsule is sealed in its own aluminum foil pocket, backed by foil on both sides. That keeps every dose away from moisture and light until you use it. This matters for metronidazole, which is sensitive to light and moisture.
A bottle is different. Every time you open it, the tablets still inside meet air, light, and humidity again. Sealed pockets also make tampering easy to see, and U.S. regulators count blister packs as tamper-evident. Blister packs cost us more than bulk bottles. The protection is worth it.
What it treats. And what it doesn’t
Where metronidazole can help
Metronidazole for fish is the go-to for internal parasites. The protozoan flagellates Hexamita and Spironucleus.1 These are behind hole-in-the-head disease and Malawi bloat, most often in cichlids and discus, with signs like appetite loss, weight loss, and white stringy feces.2 It also treats anaerobic bacterial infections. The kind that thrive without oxygen.3 When a vet confirms one of these, metronidazole can be an effective choice. The drug and dose remain the vet’s decision.
Where it’s the wrong choice
Metronidazole isn’t a general-purpose antibiotic. It doesn’t cover most gram-negative infections. The ulcers, fin rot, and body swelling behind dropsy and popeye usually need a different drug, such as a fluoroquinolone or aminoglycoside.3 Using metronidazole for the wrong problem wastes time a sick fish may not have. A vet should confirm the cause, and where possible test, rather than guessing.
Take care with your tank: remove activated carbon before dosing (it absorbs the medicine), treat in a hospital tank away from shrimp, snails, and corals, and monitor water quality. And a fish vet should confirm the cause before you treat.
Fish zole strengths and label rates by tank volume
Fish zole is sold as metronidazole tablets in a 250 mg option and a 500 mg “Forte” option. The same product also sold as Fish Zole, Fish Zole Forte, or Aqua Zole (Thomas Labs brand names), and the same molecule as human Flagyl. The quick-reference below scales a common manufacturer’s label rate of about 250 mg per 10 gallons to typical tank sizes. It is the label rate for planning, not a prescription. And for internal parasites like Hexamita, vets often prefer medicated food over in-tank dosing, since the fish must take the drug internally. Your fish veterinarian sets the actual route, dose, and duration.
| Tank volume | In-tank label rate | 250 mg tablets | 500 mg tablets |
|---|---|---|---|
| 10 gallons (38 L) | 250 mg | 1 | — |
| 20 gallons (76 L) | 500 mg | 2 | 1 |
| 40 gallons (151 L) | 1000 mg | 4 | 2 |
| 60 gallons (227 L) | 1500 mg | 6 | 3 |
Dose by the water volume of your hospital or quarantine tank, not by the number of fish. For internal-parasite cases the in-food route often works better than in-tank dosing, ask your aquatic vet which fits. The bottle label and your veterinarian always take priority over this quick reference.
Not for human use. This product is for aquarium fish only. It isn’t approved for people. Do you want to know if fish antibiotics are safe for humans? Please read our guidance. Then talk to a doctor. Do not take this product.
How your order is reviewed and shipped
Add the quantity you need to the cart (1–12). Instead of a public checkout, our pharmacy emails you to confirm your delivery address and send a secure payment link. Nothing ships until that review is complete. It is how a vet-staffed operation keeps eyes on every order and confirms it’s intended for appropriate aquarium use. We email you at each step, reserved, confirmed, payment link sent, paid, and shipped. So you never have to sit and refresh a page.
GETTING THE MOST FROM AN ANTIBIOTIC
Antibiotics work on bacterial infections, not viruses — so the first step is simply confirming that’s what your pet has. That’s what the vet review here is for: matching the right antibiotic to the actual problem, at the right dose.
If a course is prescribed, giving all of it as directed — rather than stopping early once your pet looks better — is what fully clears the infection and keeps the medicine working well for next time. You clearly want to do right by your animal; this is just the way to get the best result from it.
Consistent with WHO and CDC guidance on responsible antibiotic use.
Frequently asked questions about fish zole
What does fish zole (metronidazole) treat?
Mainly internal parasites, Hexamita and Spironucleus, behind hole-in-the-head disease and Malawi bloat. And anaerobic bacterial infections. A fish vet should confirm the cause and choose the drug and dose.
Is metronidazole good for hole-in-the-head or bloat?
Yes, these are classic metronidazole indications, most common in cichlids and discus. Because carbon absorbs the medicine, remove it before dosing, and treat in a hospital tank when you can.
Will fish zole treat dropsy or popeye?
Usually not. Dropsy and popeye are most often gram-negative infections, which metronidazole doesn’t cover well, those typically need a different drug. See our dropsy and popeye guides, and ask a vet.
Is fish zole safe for humans?
No. This product is for ornamental aquarium fish only and isn’t for human use. If you need antibiotics for a person, see a doctor or pharmacist. You can read our guidance on this.
Do I need a prescription?
This is an over-the-counter product for aquarium use. After you place an order, our pharmacy confirms your delivery details and sends a secure payment link before anything ships.
Is fish zole the same as Fish Zole Forte, Aqua Zole, or Flagyl?
Same active drug, metronidazole. Fish Zole is the 250 mg tablet and Fish Zole Forte is the 500 mg version (Thomas Labs brands); Aqua Zole is another aquarium metronidazole brand; Flagyl is the human brand name for the same molecule. This product is labeled for aquarium fish only. Dose from the label on the bottle you’ve.
Does fish zole come in 250 mg and 500 mg?
Yes, metronidazole for fish is sold as 250 mg (standard) and 500 mg (Forte) tablets. For internal parasites, ask your vet whether medicated food is a better route than in-tank dosing.
Where can I buy metronidazole for fish, do Walmart or Tractor Supply sell it?
General retailers don’t reliably carry metronidazole for fish. A dedicated vet-staffed pharmacy is the more reliable route, with clear strength labels and fresh stock. You can reserve it here and our pharmacy confirms your order before it ships.
Related guides
Swim bladder problems, often digestive rather than infection; metronidazole only fits the minority of internal-infection cases a vet confirms.
Fish dropsy, usually gram-negative, so metronidazole is typically NOT the right drug; a vet may choose a fluoroquinolone or aminoglycoside instead.
Fish popeye. The bulging-eye sign, its causes, and when antibiotics help.
Are fish antibiotics safe for humans?. A clear, no-sales answer.
How we reviewed this page
This product page is prepared under veterinary oversight following our editorial standards. The clinical statements, what metronidazole treats and where it’s not the right choice, are checked against aquatic-veterinary references and reviewed and approved by our named fish veterinarian, Dr. Julia Bauer, before publication.
Sources & References
- Merck Veterinary Manual, “Bacterial Diseases of Fish.” merckvetmanual.com (2024) Supports: gram-negative etiology of common fish infections; sensitivity-guided drug selection.
- MSU / SRAC Publication 478, “Aeromonas Bacterial Infections, Motile Aeromonad Septicemia.” srac.msstate.edu (1998) Supports: aeromonads are gram-negative; exophthalmia and related signs.
