Aquarium fish antibiotics
Fish Sulfa: Sulfamethoxazole-Trimethoprim for Fish (SMZ-TMP)
A licensed, vet-staffed pharmacy dispenses this fish sulfa. A two-drug combination. And helps you use it for the right reason. A broad folate-pathway antibiotic for ornamental fish. Intended for aquarium fish only, not for human use.
These images are for illustrative purposes only.
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.
New here? See how our free vet prescription works. You pay only for the medication; a licensed vet must approve it first. A licensed vet reviews every order, with no separate consult fee.
Quick answer: Fish sulfa is sulfamethoxazole-trimethoprim (SMZ-TMP), a combination of two drugs that work together on the same bacterial pathway. It gives broad cover, gram-positive and select gram-negative bacteria behind gill disease, fin rot, body ulcers, and Aeromonas infections. 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 sulfa is
Fish sulfa is the common name for sulfamethoxazole-trimethoprim, often shortened to SMZ-TMP, sold for aquarium fish. It isn’t one drug but two, combined in a single tablet. This product is the same combination used in human medicine, manufactured and labeled specifically for ornamental fish.
Why the two-drug combination works
Most fish antibiotics are a single drug. Fish sulfa is different: sulfamethoxazole and trimethoprim block two different, back-to-back steps in how bacteria make folate. A building block they need to grow.4 Sulfamethoxazole blocks the first step; trimethoprim blocks the next one.5 Hitting the same pathway twice makes the pair work together, which is why a vet may reach for it when broad cover is needed. It is broad rather than targeted, useful when the picture is mixed, but still a vet’s decision, not a guess.
We are a licensed, vet-staffed pharmacy, not a repackager. Our job is to sell you the right treatment, and to help you use fish sulfa for the right reason. A vet confirms the cause and the dose. That care is the whole point.
| Active ingredients | Sulfamethoxazole 800 mg + Trimethoprim 160 mg |
|---|---|
| Strength | 960 mg combined per tablet (Forte) |
| Count | 30 or 60 tablets |
| Drug class | Sulfonamide + diaminopyrimidine (folate-pathway combination) |
| 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 sulfa tablets, which are 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 fish sulfa can help
SMZ-TMP gives broad cover, gram-positive and select gram-negative bacteria.1 Vets use it for gill disease, fin rot, body ulcers, and infections linked to Aeromonas and Pseudomonas.2 Because it’s broad, it can be a reasonable choice when the exact bacteria aren’t yet known. The drug and dose remain the vet’s decision.
Where it’s the wrong choice
Fish sulfa doesn’t treat viral, fungal, or true parasitic problems. And "broad" doesn’t mean "for everything", when a vet knows the target, a more focused drug is often better: penicillin for clearly gram-positive infections, or a fluoroquinolone or aminoglycoside for the deep gram-negative infections behind many dropsy cases.3 There is also a point some sellers skip: SMZ-TMP is bacteriostatic. It slows bacteria and leans on the fish’s own immune system to finish the job, rather than killing outright. Some aquarium references consider sulfa drugs relatively weak for many fish infections, so it works best on the milder, early cases a vet has confirmed, not as a rescue for a fish that’s already crashing. A vet should confirm the cause before you treat.
Take care with your tank: remove activated carbon before dosing, treat in a hospital tank when you can, and monitor water quality. And a fish vet should confirm the cause before you treat.
Fish sulfa strengths and label rates by tank volume
Fish sulfa is sold as a sulfamethoxazole/trimethoprim tablet, commonly the 960 mg “Forte” strength. The same product also sold as Fish Sulfa, Fish Sulfa Forte, or Fix Sulfa (Thomas Labs brand names), and the same drug combination people know as human Bactrim or the brand SulfaPlex. The quick-reference below scales a common manufacturer’s label rate of about one 960 mg tablet per 20 gallons to typical tank sizes. It is the label rate for planning, not a prescription. Your fish veterinarian sets the actual dose, frequency, and duration.
| Tank volume | Label rate (SMZ-TMP) | 960 mg tablets |
|---|---|---|
| 20 gallons (76 L) | 960 mg | 1 |
| 40 gallons (151 L) | ~1920 mg | 2 |
| 60 gallons (227 L) | ~2880 mg | 3 |
| 80 gallons (303 L) | ~3840 mg | 4 |
Dose by the water volume of your hospital or quarantine tank, not by the number of fish, and pre-dissolve each tablet. The bottle label and your aquatic veterinarian always take priority over this quick reference. Because SMZ-TMP is bacteriostatic and considered relatively mild for fish, finishing the full course and keeping water quality high matter as much as the dose.
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 sulfa
What does fish sulfa (SMZ-TMP) treat in fish?
Broad bacterial infections, gill disease, fin rot, body ulcers, and Aeromonas-type infections. A fish vet should confirm the cause and choose the drug and dose.
Why is it two drugs instead of one?
Sulfamethoxazole and trimethoprim block two back-to-back steps of the bacterial folate pathway, so they work together. That is what makes the combination broad and why it’s sometimes chosen when the exact bacteria are unknown.
Should I use fish sulfa or a single-drug antibiotic?
It depends on what a vet finds. Fish sulfa is broad, useful when the picture is mixed; a targeted single drug is often better once the bacteria are known. See our penicillin and ciprofloxacin pages, and ask a vet.
Is fish sulfa 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 sulfa the same as Fish Sulfa Forte, Fix Sulfa, SulfaPlex, or Bactrim?
They are the same drug combination, sulfamethoxazole with trimethoprim. Fish Sulfa and Fish Sulfa Forte (960 mg) were Thomas Labs brand names; Fix Sulfa is a current version; SulfaPlex is a brand of the same class; and Bactrim is the human brand for the same combo. This product is labeled for aquarium fish only. Dose from the label on the bottle you’ve.
Is fish sulfa the same as "fish bactrim"?
Yes, "fish bactrim" is a nickname for sulfamethoxazole/trimethoprim sold for aquarium use, the same combination on this page. It is for ornamental fish only, not people.
Where can I buy SMZ-TMP for fish, do Walmart or Tractor Supply sell it?
General retailers don’t reliably carry aquarium SMZ-TMP. 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
Fin rot. A common infection where a vet may try a broad drug like fish sulfa on milder, early cases.
Columnaris. The fast cotton-wool disease; sulfa is broad but bacteriostatic, so a vet often prefers a faster-acting drug for severe cases.
Fish dropsy, usually gram-negative and often better matched by a fluoroquinolone or aminoglycoside; a vet decides whether a broad drug like fish sulfa fits.
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 fish sulfa treats, how the two-drug combination works, 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.
- Merck Veterinary Manual, “Antibacterial Agents (Overview of Antimicrobial Therapy).” merckvetmanual.com (2024) Supports: matching drug spectrum to the confirmed pathogen; targeted vs broad choices.
- Yan et al., “Transcriptomic Association Analysis of the Metabolic Mechanism of Sulfamethoxazole in Channel Catfish.” NCBI PMC11011017 (2024) Supports: sulfamethoxazole competitively blocks dihydrofolate synthase, halting bacterial folate synthesis.
- Case report, “Combination of Methotrexate and Trimethoprim-Sulfamethoxazole” (mechanism section). NCBI PMC5998292 (2018) Supports: SMZ blocks dihydrofolic-acid formation; TMP blocks its reduction to tetrahydrofolate (sequential two-step block).
