Ornamental aquarium and fish tank use (freshwater and saltwater)
Manufacturer
Aqua Soma Labs
Prescription
No vet prescription required (over-the-counter)
Fix Sulfa (Fish Sulfa) SMZ/TMP 960 mg is an over-the-counter (OTC) fish antibiotic for ornamental aquarium fish. This sulfamethoxazole and trimethoprim combination (SMZ/TMP) from Aqua Soma Labs is a current-production, dual-ingredient antibiotic that aquarists use to help manage suspected bacterial infections in freshwater and saltwater aquariums.
In older hobby listings and aquarium forums, this SMZ/TMP medication also goes by Fish Sulfa, Fish Sulfa Forte, FISHBIOTIC SMZ/TMP, Aqua Sulfa, and informally "fish Bactrim" or "Bactrim for fish," because Bactrim is a human brand of the same two ingredients. Fix Sulfa is the current-production replacement for the discontinued Thomas Labs Fish Sulfa Forte. Aquarists use this combination for fin and tail rot, columnaris (often presenting as cotton wool disease), skin sores and ulcer-like lesions, bacterial hemorrhagic septicemia, pop-eye (exophthalmia), cloudy eyes, and body slime issues caused by susceptible gram-negative and gram-positive bacteria such as Aeromonas, Vibrio, or Streptococcus.
SMZ/TMP (co-trimoxazole) is a dual-ingredient antibiotic that works on bacterial folate synthesis at two steps, which helps slow or stop the growth of susceptible bacteria. When used as directed for fish, sulfamethoxazole/trimethoprim works best alongside stable water parameters, strong aeration, and good husbandry. Fix Sulfa is the current-production version, and it does not require a prescription.
Fix Sulfa comes as a consistent 960 mg tablet that is easy to crush and dissolve. You can buy it online in bottles of 30 or 60 tablets with fresh expiration dates, so you can match the amount to a single quarantine tank or a multi-tank fish room.
Start treatment at the first signs of bacterial stress, and treat affected fish in a hospital or quarantine tank whenever possible. Watch for the symptoms below.
Fish Symptoms Aquarists Treat With Fix Sulfa
Fin and tail rot (frayed or rotting fins)
Skin sores, ulcer-like lesions, and red streaks
Pop-eye (bulging eyes) and cloudy eyes
Columnaris, often with a cotton wool look
Bacterial hemorrhagic septicemia and body slime issues
Lethargy, rapid breathing, or loss of appetite that clean water does not fix
Order before 1:30 p.m. CT, Monday through Friday, for same-day dispatch. Fix Sulfa ships from our U.S. warehouse via USPS or UPS.
Important Notice: This product is intended for aquarium and ornamental fish only. It is not for human consumption or for animals used for food. We do not sell or recommend this product for human use.
Fish Sulfa Forte vs Fix Sulfa: What Changed?
If you previously purchased Thomas Labs Fish Sulfa Forte, FISHBIOTIC SMZ/TMP or Aqua Sulfa, here is what has changed and what remains the same:
Same Active Ingredients Fix Sulfa contains sulfamethoxazole 800 mg and trimethoprim 160 mg per tablet (SMZ/TMP 960 mg), the same dual-ingredient formulation found in legacy products like Thomas Labs Fish Sulfa Forte, FISHBIOTIC SMZ/TMP, and Aqua Sulfa.
Updated Branding Aqua Soma Labs makes Fix Sulfa as a current-production replacement for the discontinued Thomas Labs Fish Sulfa Forte and other legacy SMZ/TMP fish antibiotic brands. The active ingredients and intended use are the same.
Same Tablet Form Each tablet delivers a consistent 960 mg dose in a format that is easy to crush and dissolve for aquarium use, following the same scored-tablet approach aquarists are familiar with from Fish Sulfa Forte.
Flexible Bottle Sizes Available in 30-count and 60-count bottles with fresh expiration dates, giving hobbyists options for short treatment courses, quarantine setups, or multi-tank fish rooms.
Benefits of Fix Sulfa for Fish
Dual-Action SMZ/TMP Combination
Fix Sulfa combines sulfamethoxazole and trimethoprim (SMZ/TMP), a paired approach that interferes with bacterial folate production at two steps to help manage susceptible bacterial issues when used as directed.
No Prescription Required
Fix Sulfa is available over the counter (OTC) for aquarium and ornamental fish. You can order online and have it shipped directly without needing a veterinary prescription.
Broad-Spectrum Support for Common Aquarium Problems
Aquarists commonly reach for fish sulfa tablets when dealing with symptoms associated with fin and tail rot, columnaris-type presentations, cotton wool disease, ulcer-like lesions, pop-eye, and other persistent bacterial signs in ornamental freshwater and saltwater fish.
Helps With External and Internal Signs of Bacterial Stress
SMZ/TMP tablets are often used in a hospital tank for visible skin sores, fin damage, and hemorrhagic septicemia, and in situations where fish show lethargy or reduced appetite alongside water-quality corrections and supportive care.
Consistent 960 mg Tablet Strength
Each tablet provides a consistent 960 mg format, making it easier to plan repeatable dosing across different tank sizes. Available in 30- or 60-tablet bottles.
Intended for Ornamental Fish Use
Fix Sulfa is intended for aquarium and ornamental fish use when used as directed, and the tablet format offers a straightforward option for hobbyists who prefer predictable, countable dosing.
Helpful for Quarantine and Hospital Tanks
Treating in a separate hospital or quarantine tank can help you observe fish more closely, control dosing more precisely, and reduce unnecessary exposure for healthy tank mates during treatment.
Dosage Guidelines & Instructions
These steps follow the Fix Sulfa 960 bottle label directions for SMZ/TMP use in ornamental aquarium fish.
Dosage Quick Reference: Dissolve one (1) dose (one tablet) and add it for every 40 gallons (151 L) of water to be treated. Repeat every 24 hours with a partial water change between treatments for 5-10 days. Discontinue use if no improvement is noted after 5 days.
Before You Dose:
Calculate actual water volume (account for substrate, decor, and fill level).
Use a hospital/quarantine tank when possible for better control and observation.
Remove activated carbon and other chemical media; turn off UV sterilizers during treatment.
Increase aeration and surface agitation; keep temperature stable.
Directions for Use:
Dissolve and Add the Dose
Fully dissolve one (1) dose (one tablet) in a small cup of aquarium water, then add the dissolved dose to the treatment tank. Dose at one tablet per 40 gallons (151 L) of water to be treated.
Repeat Every 24 Hours
Repeat dosing every 24 hours. Perform a partial water change between treatments throughout the course.
Complete the Course and Watch for Progress
Continue treatment for 5-10 days. Watch appetite, swimming behavior, fin condition, and any signs of stress. Discontinue use if no improvement is noted after 5 days. If fish appear worse or show severe stress, stop treatment and seek aquatic veterinary advice.
Post-Treatment Clean Up
After the course is complete, perform a partial water change and resume normal filtration. Replace activated carbon or chemical media if you removed it, and keep monitoring water parameters for several days to confirm stability.
Fix Sulfa Dosing by Aquarium Size
Tank volume
Fix Sulfa 960 dose
How to measure
10 gallons (38 L)
1/4 tablet
Split a scored tablet, or add 1/4 of one dissolved tablet
20 gallons (76 L)
1/2 tablet
Split a scored tablet, or add 1/2 of one dissolved tablet
40 gallons (151 L)
1 tablet (label standard)
Dissolve one whole tablet
80 gallons (303 L)
2 tablets
Dissolve two whole tablets
120 gallons (454 L)
3 tablets
Dissolve three whole tablets
The Fix Sulfa 960 label states one (1) dose (one tablet) per 40 gallons (151 L), which is about 24 mg of combined sulfamethoxazole and trimethoprim per gallon. The 40-gallon row is the exact label instruction; the other rows scale that same rate. For tanks under 40 gallons you can split a scored tablet, or dissolve one tablet in a measured cup of tank water and add the matching fraction for more accuracy. Dose by your treatment tank's water volume, not the number of fish, repeat once every 24 hours for 5 to 10 days, and follow the bottle label or your aquatic veterinarian over this quick-reference chart.
What to Expect During Treatment
Days 1 to 2
Fish may hide or eat less, and the water can look slightly cloudy. This is normal early on. Keep dosing on schedule and keep aeration strong.
Days 3 to 5
Watch for signs it is working: sores and red patches stop spreading, appetite starts to return, and breathing steadies. If there is no change by day 5, or the fish look worse, stop and ask an aquatic veterinarian.
Days 5 to 10
Even if the fish are improving, finish the full course so the infection does not return. Afterward, run a partial water change and put activated carbon back in to clear any leftover medication.
What You Need for a Treatment Course
A little setup before the first dose saves trouble later.
A separate hospital or quarantine tank, so you are not medicating the whole display.
A heater and an air stone or sponge filter for steady temperature and strong aeration.
An ammonia and nitrite test kit to keep tabs on water quality.
A dechlorinator for the water changes between doses.
A small net and a clean cup for gentle transfer and measuring the dose.
Pro Tip: Hospital tank treatment helps protect the display aquarium, keeps dosing consistent, and makes it easier to keep ammonia and nitrite at 0 during recovery.
How It Works
Active Ingredients (each tablet)
Sulfamethoxazole 800 mg and Trimethoprim 160 mg (SMZ/TMP, also known as co-trimoxazole). This paired combination works at two steps in bacterial folate synthesis, which helps slow the growth of susceptible bacteria when used as directed. The chemical identity and properties of these active ingredients are documented by NIH PubChem: sulfamethoxazole, CID 5329 and trimethoprim, CID 5578.
Inactive Ingredients
Tablet excipients that help keep the active ingredients stable and allow the tablet to disperse properly when dissolved and added to aquarium water as directed. Inactive ingredients are not specified in the active-ingredient panel.
Side Effects & Safety Tips
Individual fish can respond differently to treatment. For best results and safety, follow the label directions and monitor fish and water quality closely throughout the course.
Possible Side Effects in Fish
Temporary reduced appetite, mild lethargy or hiding behavior, and occasional water cloudiness during treatment. If fish show severe distress such as rapid gill movement, gasping at the surface, loss of balance, or lying on the bottom, stop treatment, increase aeration, and seek aquatic veterinary advice.
Stop Treatment If
Fish show severe stress, rapid gill movement, gasping at the surface, loss of balance, or worsening symptoms. Increase aeration immediately and consult an aquatic veterinarian if signs do not stabilize.
Water Quality and Biofilter Awareness
Keep aeration strong and monitor ammonia and nitrite closely during treatment. If parameters rise, perform partial water changes as needed to keep the treatment tank stable.
Filtration During Treatment
Remove activated carbon and other chemical filtration media before dosing. Turn off UV sterilizers during treatment. Resume normal filtration after the course is complete.
Invertebrates and Display Tanks
If your system includes shrimp, snails, corals, or other invertebrates, treat fish in a separate hospital or quarantine tank whenever possible to reduce risk to the display ecosystem.
Important Warnings & Precautions
Aquarium Use Only
Fix Sulfa is intended for aquarium and ornamental fish only. It is not for human use and not for animals raised for food.
Not for Food Fish
Do not use on fish intended for human consumption. This product is manufactured, packaged, and labeled as an OTC fish antibiotic for ornamental aquarium fish only.
Handling and Sulfa Allergy
If you are allergic to sulfa drugs (sulfonamides), avoid contact with the dissolved medication, wash your hands after handling, and wear gloves if needed. Keep the capsule contents off skin and out of eyes.
Keep Out of Reach
Store Fix Sulfa out of reach of children and pets. This is a fish antibiotic and should only be used as directed on the product label.
Storage
Keep the container tightly closed and store in a cool, dry place, away from direct sunlight and moisture. Check the expiration date and do not use tablets past it.
Discontinue If No Improvement
If no improvement is noted after 5 days of treatment, discontinue use as stated on the label and re-evaluate the diagnosis or seek aquatic veterinary advice.
Do Not Mix Medications
Avoid combining Fix Sulfa with other antibiotics or medications unless the product label explicitly allows it or an aquatic veterinarian advises it.
Shipping & Returns
Shipping Policy: Orders ship from our U.S. warehouse within 1 business day via USPS or UPS, with same-day dispatch on orders placed before 1:30 p.m. CT (Monday through Friday). For full shipping details, please see our shipping policy.
Refund Policy: If there is an issue with your order, you may be able to return or exchange items within 30 days according to our store guidelines. Before sending anything back, please review our refund policy carefully.
Frequently Asked Questions
What is Fix Sulfa used for?
Fix Sulfa (SMZ/TMP) is used by aquarists to help manage susceptible bacterial issues in ornamental aquarium fish when used as directed. It is commonly chosen for symptoms associated with fin and tail rot, ulcer-like lesions, pop-eye, and other persistent bacterial signs.
What is fish sulfa forte?
Fish Sulfa Forte was a Thomas Labs SMZ/TMP fish antibiotic (sulfamethoxazole and trimethoprim) for ornamental aquarium fish. Thomas Labs discontinued it, and Fix Sulfa by Aqua Soma Labs is the current-production version with the same two active ingredients in a 960 mg tablet.
What is the difference between Fish Sulfa Forte and Fix Sulfa?
Fish Sulfa Forte (Thomas Labs), FISHBIOTIC SMZ/TMP, and Aqua Sulfa are older SMZ/TMP fish antibiotic brands. Thomas Labs discontinued Fish Sulfa Forte, and Fix Sulfa by Aqua Soma Labs is the current-production version. It contains the same two active ingredients (sulfamethoxazole 800 mg and trimethoprim 160 mg) in the same 960 mg tablet format.
Is Fix Sulfa the same as "fish Bactrim"?
"Fish Bactrim" and "Bactrim for fish" are common hobbyist names for SMZ/TMP fish antibiotics like Fix Sulfa, because Bactrim is a human brand of the same two ingredients (sulfamethoxazole and trimethoprim). Fix Sulfa is labeled strictly for ornamental aquarium fish and must never be taken by humans or given to animals raised for food.
Is Fix Sulfa a replacement for Aqua-SMZ/TMP?
Yes. If you previously purchased Aqua-SMZ/TMP or similar legacy SMZ/TMP products for ornamental fish, Fix Sulfa provides the same active ingredient combination (sulfamethoxazole and trimethoprim, 960 mg per tablet) under updated branding from Aqua Soma Labs.
Do I need a veterinary prescription to buy Fix Sulfa?
No. Fix Sulfa is available as an over-the-counter (OTC) fish antibiotic for ornamental aquarium fish. You can order online without a veterinary prescription.
Where can I buy SMZ/TMP fish antibiotic tablets online?
You can buy Fix Sulfa (SMZ/TMP 960 mg) directly from Fine Pet Health. Orders placed before 1:30 p.m. CT (Mon-Fri) typically ship the same business day from our U.S. warehouse.
What are the active ingredients in each Fix Sulfa tablet?
Each tablet contains sulfamethoxazole 800 mg and trimethoprim 160 mg (SMZ/TMP), a paired combination also known as co-trimoxazole.
What is the Fix Sulfa dosage for fish?
Follow your bottle label directions. The Fix Sulfa 960 label instructions state: dissolve and add one (1) dose (one tablet) per 40 gallons (151 L) of water to be treated, repeat every 24 hours with a partial water change between treatments for 5-10 days, and discontinue use if no improvement is noted after 5 days.
How much Fix Sulfa should I use per gallon?
The Fix Sulfa 960 label directs one (1) dose (one tablet) per 40 gallons (151 L) of water to be treated, which is about one tablet per 40 gallons. Dissolve the dose, add it to the treatment tank, and repeat every 24 hours with a partial water change between doses for 5-10 days. Measure the tank volume first.
How long should I treat with Fix Sulfa?
For fish, the Fix Sulfa 960 label directs a course of 5-10 days, dosing once every 24 hours with a partial water change between treatments. Discontinue use if no improvement is noted after 5 days, then re-check water quality and the likely cause before continuing.
Should I treat in the main tank or a hospital tank?
A hospital or quarantine tank is recommended whenever possible. It makes dosing easier, helps you monitor fish closely, and can reduce disruption to the display aquarium's biofilter and tank mates.
Do I need to remove activated carbon or turn off UV during treatment?
Yes. Activated carbon and other chemical media can remove medications from water, and UV sterilizers may reduce medication availability. Remove carbon/chemical media and turn off UV during treatment, then resume normal filtration after the course is complete.
Will Fix Sulfa harm beneficial bacteria in the aquarium?
Any antibiotic treatment can affect beneficial filter bacteria, especially if used in the main tank. Monitor ammonia and nitrite closely, maintain strong aeration, and perform partial water changes as needed. Using a hospital tank helps protect the main biofilter.
Can Fix Sulfa treat fungal, viral, or parasitic infections?
Fix Sulfa (SMZ/TMP) is an antibiotic intended for susceptible bacterial issues. It is not designed for fungal, viral, or parasitic problems. If you suspect fungus, parasites, or mixed causes, consult an aquatic veterinarian or experienced aquarist to choose an appropriate treatment plan.
Is Fix Sulfa safe for betta fish and small aquariums?
Bettas and other small-tank fish can be treated with Fix Sulfa, but accurate dosing is especially important in smaller volumes. Measure your actual water volume carefully and use a hospital container for best results. Monitor closely for any signs of stress.
Does Fix Sulfa work in saltwater aquariums?
SMZ/TMP tablets can be used in saltwater setups when treating ornamental fish. If your system includes corals, invertebrates, or a refugium, treat fish in a separate hospital or quarantine tank to reduce risk to the display ecosystem.
Can I combine Fix Sulfa with other fish medications?
Avoid combining Fix Sulfa with other antibiotics or medications unless the product label explicitly allows it or an aquatic veterinarian advises it. Mixing medications can increase stress on fish and make it harder to identify what is working.
What if my fish are not improving after treatment?
If no improvement is noted after 5 days, discontinue use as stated on the label. Re-check water parameters, confirm the likely cause of symptoms, and seek aquatic veterinary advice or consider an alternative approach based on a more accurate diagnosis.
How should I store and dispose of Fix Sulfa tablets?
Keep the container tightly closed and store in a cool, dry place away from direct sunlight. Keep out of reach of children and pets. For disposal, follow your local guidelines for unused medications. Do not flush tablets or pour dissolved medication into natural waterways.
Bottom Line
Fix Sulfa is an SMZ/TMP 960 mg fish antibiotic (sulfamethoxazole 800 mg plus trimethoprim 160 mg) for susceptible bacterial problems in ornamental aquarium fish. It is the current-production replacement for the discontinued Thomas Labs Fish Sulfa Forte, and the same medication hobbyists call "fish Bactrim." Dose by water volume at one tablet per 40 gallons (151 L), treat for 5 to 10 days in a hospital tank with a partial water change between doses, and reassess at day 5. It will not help with fungal, viral, or parasitic problems, and it is not for human use or for animals raised for food. If you are unsure whether the cause is bacterial, talk to an aquatic veterinarian first.
Ready to start? Choose the bottle size that fits your setup above, or compare the rest of the Fix line below.
Sources
Key references used to check the drug facts on this page. Always follow the directions on your product's label.
University of Florida IFAS Extension. Yanong, R.P.E. Use of Antibiotics in Ornamental Fish Aquaculture (FA084). edis.ifas.ufl.edu
More Fish & Bird Antibiotics
Fix Mox (Amoxicillin) 250/500mg A broad-spectrum penicillin-type fish antibiotic for fin rot, dropsy, ulcers and other bacterial infections. Available in 30-count and 100-count capsules. Shop Fix Mox
Fix Zole (Metronidazole) 250/500mg Targets anaerobic bacteria and protozoa including Hexamita and hole-in-the-head disease in ornamental fish. Available in 60-count bottles. Shop Fix Zole
Fix Doxy (Doxycycline) 100mg A tetracycline-class fish and bird antibiotic effective against columnaris, respiratory infections and Mycoplasma. Available in 30-count and 60-count tablets. Shop Fix Doxy
Fix Zithro (Azithromycin) 250mg A macrolide fish and bird antibiotic for fin rot, columnaris, gill disease and avian respiratory concerns. Available in 12-count and 30-count tablets. Shop Fix Zithro
Fix Cin (Clindamycin) 150mg A lincosamide fish antibiotic commonly used for gram-positive bacterial infections including skin and soft tissue concerns in ornamental fish. Shop Fix Cin
Fix Pen (Penicillin) 500mg A penicillin-type fish and bird antibiotic for gram-positive bacterial infections in ornamental fish and non-food pet birds. Shop Fix Pen
Fix Flex (Cephalexin) 250/500mg A first-generation cephalosporin fish and bird antibiotic for fin rot, ulcers and bacterial skin infections in ornamental fish. Shop Fix Flex
Fix Flox (Ciprofloxacin) 500mg A fluoroquinolone fish and bird antibiotic for gram-negative bacterial infections including fin rot, ulcers, columnaris and septicemia in ornamental fish. Shop Fix Flox
Guides and treatment tips Visit the Fish & Bird Antibiotics Blog for practical dosing guides, water-care advice and treatment tips.
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About Fine PetHealth and How We Verify Our Antibiotic Guides
Who is responsible for this page: Fine PetHealth Editorial Team.
Verification snapshot: This guide is written from the current product label for this item and reviewed for accuracy and consistency before publishing.
How we verify product information (our checklist):
1) Label-first accuracy: Dosing, duration, storage, and warnings are taken from the current product label for this item.
2) Ingredient validation: Active ingredient names are cross-checked against reputable references (for example, NIH PubChem) to reduce the risk of labeling mistakes.
3) Consistency checks: We review for internal consistency (the same dosing and storage language across sections) and remove wording that could overpromise results.
4) Updates: We update pages when label directions, packaging, or manufacturer information changes.
What we don’t do: We don’t invent diagnoses, and we don’t create dosing instructions beyond what the product label states for this item’s intended use.
See something that doesn’t match your bottle? Please contact us so we can review and update the page:
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Educational use: Our content is for informational purposes and is not a substitute for diagnosis or treatment from an aquatic or avian veterinarian.
Tried to order fish meds through the other guy, but their web site wouldn't work, so I tried fine pets, it was fast, easy, and no problems, definitely got my business from now on.
Hi Daniel, we really appreciate the awesome review! It’s great to hear that ordering your Fix Sulfa with us was a breeze after having trouble elsewhere. Thanks for choosing us, and we can't wait to see you for your next order!
D
David McConnaughey (Las Vegas, NV)
Fix Sulfa (SMZ/TMP) 960mg 30/60ct Fish Antibiotic
Dear David, thank you immensely for leaving a 5-star review for your recent Fix Sulfa purchase! We are so glad to see that you are fully satisfied with your order. Your support is incredibly valuable to us, and we can't wait to assist you again the next time you need to restock!
J
Janice Amelunk (Jamestown, TN)
Product as described
Would buy from again!
J
Jackie Eaten (Chickamauga, GA)
Fast an accurate
Very fast service an always accurate
A
Amy Patterson (Jacksonville, FL)
Aqua-SMZ/TMP
Item made it on time, and solved my fish bacterial problem.
