Ornamental aquarium and fish tank use, plus non-food pet birds
Manufacturer
Aqua Soma Labs
Prescription
No vet prescription required (over-the-counter)
Fix Pen (Fish Pen) is an over-the-counter (OTC) fish penicillin antibiotic for ornamental aquarium fish and non-food pet birds. Each tablet delivers a fish penicillin 500mg dose of Penicillin V Potassium (Penicillin VK), made by Aqua Soma Labs in the United States and sold in bottles of 30 or 60 tablets with fresh expiration dates.
Aquarists may know this product as Fish Pen, Fish Pen Forte, Aqua Pen or FISHBIOTIC Penicillin. Those were Thomas Labs brand names, and Fix Pen is the current replacement now that the Thomas Labs line is discontinued. Keepers usually reach for penicillin tablets for fish when they see problems such as fin rot, tail rot, infected wounds, pop-eye (bulging eyes), or columnaris-type lesions (sometimes called cotton-wool disease), where a bacterial cause is suspected.
Penicillin V is a beta-lactam antibiotic. It interferes with bacterial cell wall formation and can stop susceptible gram-positive bacteria from multiplying. Fix Pen is the fish penicillin 500mg tablet many hobbyists use as a penicillin for fish tank infections, and the same fish pen penicillin tablets are used as penicillin for birds under avian veterinary guidance. It targets gram-positive and some gram-negative bacteria in ornamental aquarium use.
Each tablet contains 500 mg, and bottles come in 30 or 60 tablets. The 30-count suits a single hospital tank or occasional use; the 60-count fits multi-tank setups or aviary needs. You can buy Fix Pen online with fresh expiration dates and U.S. shipping.
Start treatment early, at the first signs of a suspected bacterial problem. The signs are not the same for fish and birds, so use the lists below to know what to watch for.
Soft tissue infections where a vet suspects bacteria
Fluffed-up feathers, lethargy or inactivity
Decreased appetite
For birds, only reach for Fix Pen when an avian veterinarian has recommended penicillin for a specific problem. Catching an infection early, paired with clean water and good husbandry, gives your fish and birds the best chance to recover.
Husbandry still does most of the work. Stable water and good feeding prevent more problems than any antibiotic fixes. Penicillin does nothing for viral issues, and using fish antibiotics like penicillin when they are not needed can reduce how well they work later.
Order online: in-stock orders placed before 1:30 p.m. CT, Monday through Friday, ship the same day from our U.S. warehouse by USPS or UPS.
Important Notice: This product is intended for aquarium and ornamental fish and pet birds 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 Pen Forte vs Fix Pen: What Changed?
If you used Thomas Labs Fish Pen, Fish Pen Forte, Aqua Pen or FISHBIOTIC Penicillin, here is what is the same and what is different:
Same Active Ingredient Fix Pen contains the same 500 mg Penicillin V Potassium (Penicillin VK) per tablet that you used in Fish Pen and Fish Pen Forte. The formulation targets the same bacteria.
New Manufacturer Fix Pen is made by Aqua Soma Labs, which replaced the discontinued Thomas Labs line. Aqua Soma Labs manufactures it in the United States.
Same Tablet Form Fix Pen uses the same tablet format as the original Fish Pen, so it is easy to dissolve in aquarium water or crush for precise dosing in a hospital tank.
Available in 30-Count and 60-Count Bottles Pick the size that fits your setup: a 30-count bottle for a single hospital tank or occasional use, or a 60-count bottle for larger collections, multi-tank operations or aviary needs.
Benefits of Fix Pen for Fish & Birds
Targeted Gram-Positive Bacterial Coverage
A classic penicillin choice when a gram-positive bacterial component is suspected in ornamental fish, especially when the signs look like fin and tail rot, mouth rot, sores, pop-eye or cloudy eyes.
No Prescription Required
Fix Pen is available over the counter for aquarium and avian use, so you can start treatment without waiting for a prescription. For birds, veterinary guidance is still strongly recommended to set the correct dose and duration.
Consistent 500 mg Tablets
Each tablet delivers a reliable 500 mg dose of Penicillin V Potassium, so it is easy to scale from a single betta or quarantine tank to larger community aquariums.
Common Aquarium Use Cases
Often chosen when the goal is to target gram-positive and some gram-negative bacteria in ornamental fish. In aquarium care discussions, penicillin comes up for flexibacter-type issues, infected wounds and worsening fin damage when a bacterial component is suspected.
Freshwater and Saltwater Compatible
Made for ornamental aquarium use in both freshwater and saltwater setups when used as directed. A separate hospital tank is still recommended when possible to protect the main display system.
Quarantine-Friendly and Biofilter Aware
Penicillin may affect beneficial bacteria and can be risky for invertebrates like shrimp and snails. A separate treatment tank protects the main display and makes water-quality monitoring easier during the course.
Ideal for Hospital Tanks and Quarantine
Using Fix Pen in a separate hospital tank protects the main aquarium's biofilter and reduces exposure for healthy tank mates, so you can focus treatment where it is needed.
Dosage Guidelines & Instructions
Quick dosing summary for fish: Use 1 tablet per 20 gallons (76 L), repeat every 24 hours, and do a partial water change between doses. Continue for 5 to 10 days, and discontinue if no improvement is noted after 5 days.
Directions for Use:
Set Up a Separate Treatment Area
When possible, treat fish in a hospital or quarantine tank. This helps limit exposure for healthy tank mates and makes it easier to monitor results and water quality. Remove activated carbon and turn off UV sterilizers during treatment, as they can remove the medication from the water.
Dose for Fish Aquariums
Dissolve one (1) 500 mg tablet in a small cup of aquarium water first, then add the dissolved dose for every 20 gallons (76 L) of water to be treated. Repeat every 24 hours, and make a partial water change between treatments.
Duration and When to Stop
Continue treatment for 5 to 10 days. If no improvement is noted after 5 days, discontinue use and consider consulting an aquatic professional for next-step options. Complete the full recommended course even if fish appear better, as stopping early can allow symptoms to return.
Maintain Water Quality During Treatment
Keep strong aeration and closely monitor water parameters (especially ammonia and nitrite). Because penicillin may affect beneficial bacteria and can be risky for invertebrates like shrimp and snails, a separate treatment tank and close observation are strongly recommended.
Pet Birds (Under Veterinary Guidance Only)
For non-food pet birds, use penicillin only under avian veterinary guidance for dosing, frequency and duration. Your vet can advise you on the correct mg-per-kg dose and decide whether to treat a single bird or an entire flock.
Fix Pen Dosing by Aquarium Size
Tank volume
Penicillin per dose
500 mg tablets
10 gallons (38 L)
250 mg
half a tablet (split)
20 gallons (76 L)
500 mg
1
40 gallons (151 L)
1000 mg
2
60 gallons (227 L)
1500 mg
3
75 gallons (284 L)
about 1875 mg
about 4
These amounts follow the label guideline of one 500 mg tablet per 20 gallons (76 L), roughly 25 mg per gallon. Dose by the water volume of your treatment tank, not by the number of fish, give one dose every 24 hours for 5 to 10 days, and make a partial water change between doses. Splitting a tablet by hand is only approximate, so for tanks under 20 gallons it is better to dissolve a whole tablet in a measured cup of tank water and add the matching fraction. University of Florida IFAS Extension notes that completing the antibiotic for the recommended number of days matters as much as choosing the right one (UF IFAS, FA084). The bottle label or your aquatic veterinarian takes priority 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
Look 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 professional.
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.
Fix Pen treatment timeline: a typical 5 to 10 day penicillin course for aquarium fish, with a reassessment point at day 5.
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: Dose by the actual treated water volume, not the tank's advertised size, since gravel, decorations and equipment reduce usable volume. Daily observation and stable water quality matter as much as the dose.
How This Medication Works
Active ingredient: Penicillin V Potassium (Penicillin VK) 500 mg
Penicillin V is a beta-lactam antibiotic that works by targeting bacterial cell wall formation. It binds to penicillin-binding proteins (PBPs) involved in peptidoglycan cross-linking, which can help control susceptible gram-positive bacteria and some gram-negative bacteria when used as directed in ornamental fish care. The chemical identity and properties of Penicillin V are documented by NIH PubChem (CID 6869).
Inactive ingredients
Tablet excipients that help keep the active ingredient stable on the shelf and allow the tablet to disperse and dissolve properly when added to aquarium water or treatment setups.
Side Effects & Safety Tips
When used as directed, Fix Pen is typically well tolerated in ornamental fish. As with any antibiotic, unnecessary use, incorrect dosing or combining treatments can increase stress and reduce future effectiveness.
Fish
Some fish may show temporary stress, reduced appetite or hiding behavior during treatment. If you notice severe distress such as rapid gill movement, loss of balance, gasping at the surface or fish lying on the bottom, stop treatment, improve aeration, perform a partial water change and seek aquatic veterinary or professional guidance.
Birds
For non-food pet birds, use penicillin only under avian veterinary guidance. If your bird becomes unusually lethargic, stops eating or appears unwell, discontinue use and contact your avian veterinarian immediately.
Invertebrates
Do not use Fix Pen in display tanks that contain shrimp, snails or other invertebrates, as penicillin may harm them or disrupt the tank ecosystem. Whenever possible, move affected fish to a separate hospital tank.
Safe use in your system
Treat fish in a hospital or quarantine tank whenever you can, remove activated carbon and turn off UV sterilizers during treatment, and monitor ammonia, nitrite and temperature closely. Do not mix Fix Pen with other antibiotics unless advised by a veterinarian.
Important safety warning
Fix Pen is intended for ornamental aquarium fish and non-food pet birds only. It is not for human use and not for animals raised for food. Using fish or bird antibiotics in people instead of prescribed medicine can lead to treatment failure, serious side effects and antibiotic resistance. Store tablets in a cool, dry place away from direct sunlight and keep out of reach of children and pets. Always follow the directions on the label.
Shipping & Returns
Shipping Policy: Orders ship from our U.S. warehouse within 1 business day via USPS or UPS, with same-day dispatch on in-stock 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 Pen used for?
Fix Pen is a fish penicillin tablet (500 mg) used to help manage suspected bacterial problems in ornamental aquarium fish, especially gram-positive infections such as fin rot, tail rot, infected wounds, pop-eye and columnaris-type lesions. For non-food pet birds, use penicillin only under avian veterinary guidance for dosing and treatment decisions.
What is the difference between Fish Pen Forte and Fix Pen?
Fish Pen and Fish Pen Forte were brand names used by Thomas Labs for their penicillin fish antibiotic tablets. After Thomas Labs discontinued these products, Aqua Soma Labs introduced Fix Pen as a direct replacement. Fix Pen contains the same 500 mg Penicillin V Potassium per tablet and is intended for the same ornamental fish and pet bird applications.
Is Fix Pen a replacement for FISHBIOTIC Penicillin or Aqua Pen?
Yes. If you previously used FISHBIOTIC Penicillin, Aqua Pen or any other 500 mg penicillin fish antibiotic tablet, Fix Pen from Aqua Soma Labs is a direct replacement with the same active ingredient (Penicillin V Potassium 500 mg per tablet) and is available in 30-count and 60-count bottles.
Do I need a veterinary prescription to buy Fix Pen?
No. Fix Pen is sold over the counter as a fish and bird antibiotic, and no veterinary prescription is required to purchase it. However, for pet birds, professional veterinary guidance is strongly recommended to determine the correct mg-per-kg dose and treatment duration.
Where can I buy penicillin fish antibiotic tablets online?
You can buy Fix Pen, a fish penicillin 500mg tablet, directly from Fine Pet Health. We ship penicillin for fish and birds from our U.S. warehouse with same-day dispatch on in-stock orders placed before 1:30 p.m. CT, Monday through Friday, by USPS or UPS.
No. Fix Pen is not a human Pen VK alternative. While it contains the active ingredient Penicillin V Potassium, this product is manufactured, packaged and labeled strictly as an over-the-counter penicillin for ornamental aquarium fish and non-food pet birds only. It must never be taken by humans, prescribed to yourself or given to animals raised for food.
What is the active ingredient in each Fix Pen tablet?
Each Fix Pen tablet contains 500 mg Penicillin V Potassium (Penicillin VK). Penicillin V is a beta-lactam antibiotic that works by interfering with bacterial cell wall formation, helping control susceptible gram-positive bacteria when used as directed.
What is the Fix Pen dosage for fish?
Follow the label directions: add one (1) 500 mg tablet per 20 gallons (76 L) of water to be treated, repeat every 24 hours and make a partial water change between doses. Continue for 5 to 10 days and discontinue use if no improvement is noted after 5 days.
How long should I treat with Fix Pen?
Label directions recommend treatment for at least 5 days but no more than 10 days. If there is no improvement after 5 days, discontinue treatment and reassess husbandry and next-step options with an aquatic professional.
Should I treat in the main tank or a hospital tank?
A separate hospital or quarantine tank is strongly recommended. Antibiotics may impact beneficial bacteria and can be risky for shrimp, snails, corals and other invertebrates, so dosing a display tank or reef setup is generally not recommended. A small, dedicated hospital tank with a heater, air stone and no substrate makes dosing and water changes easier.
Do I need to remove activated carbon or turn off UV during treatment?
Yes. Activated carbon and some chemical filtration media can remove medications from the water. Remove them during treatment and reinstall after the course is finished. Many keepers also temporarily turn off UV sterilizers during dosing to keep medication levels consistent.
Will Fix Pen harm beneficial bacteria in the aquarium?
It can. Any antibiotic, including penicillin, may affect beneficial filter bacteria if used in the main tank. Using a hospital tank, monitoring ammonia and nitrite and performing partial water changes helps protect the biofilter while treatment is underway.
Can Fix Pen be used in saltwater aquariums?
Yes. Fix Pen can be used in both freshwater and saltwater hospital tanks. Dissolve the tablet following the standard dosage (one 500 mg tablet per 20 gallons) and maintain stable salinity, temperature and water quality parameters throughout the treatment course.
Is Fix Pen safe for betta fish and small aquariums?
Yes. Fix Pen can be used in small hospital tanks for bettas and other individual fish. Because bettas are often kept in tanks under 20 gallons, you may need to split a tablet to achieve the correct dose (one 500 mg tablet per 20 gallons). Dissolve the portion in a small cup of tank water before adding it to the hospital tank.
Can I use Fix Pen in a planted tank or with shrimp and snails?
It is generally considered compatible with most planted tanks, but penicillin may harm shrimp, snails and other invertebrates. When possible, move affected fish to a separate hospital tank to reduce plant and biofilter exposure during treatment and protect invertebrates.
Can Fix Pen treat fungal, viral or parasitic infections?
No. Fix Pen is a penicillin antibiotic that targets certain bacterial infections only. It is not effective against fungi, viruses or parasites such as ich, flukes or worms. For fungal, parasitic or viral problems, work with an aquatic or avian veterinarian to choose an appropriate treatment.
Can I combine Fix Pen with other fish medications?
Combining antibiotics or mixing Fix Pen with antifungal or antiparasitic medications is generally not recommended unless specifically advised by an aquatic veterinarian. Using multiple medications at once can stress fish, cause unpredictable interactions and make it harder to identify which treatment is working.
Can I use Fix Pen for dogs, cats or chickens?
Fix Pen is labeled for ornamental aquarium fish and non-food pet birds only. It is not formulated, tested or approved for dogs, cats, chickens or other mammals and food animals. People sometimes look up fish penicillin for dogs, but if your dog or cat needs penicillin, a licensed veterinarian should prescribe the correct product and dose for that species.
Can I use Fix Pen as penicillin for pet birds?
Fix Pen contains Penicillin V Potassium, the same penicillin class used in avian care, so it can serve as a penicillin for birds option. Bird dosing is different from fish dosing and depends on the species and body weight, so use bird penicillin only under the direction of an avian veterinarian who can confirm a susceptible infection and set the dose and duration. Fix Pen is intended for non-food pet birds, not poultry raised for meat or eggs.
How should I store and dispose of Fix Pen tablets?
Store tablets in their original container in a cool, dry place away from heat, moisture, children and pets. Check the expiration date on the packaging before use. For disposal, a local medicine take-back program is the best option. If a take-back option is not available, follow FDA/EPA guidance for household medication disposal and avoid flushing unless the label specifically instructs it.
Bottom Line
Fix Pen is a fish penicillin 500mg tablet (Penicillin V Potassium) for gram-positive bacterial problems in ornamental aquarium fish, and a vet-directed penicillin option for non-food pet birds. It is the current replacement for the discontinued Thomas Labs Fish Pen and Fish Pen Forte. Dose by water volume at one 500 mg tablet per 20 gallons, treat for 5 to 10 days in a hospital tank, 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, or you are treating a bird, talk to an aquatic or avian veterinarian first. Ready to start? Choose the 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). ask.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 Sulfa (SMZ/TMP) 960mg A sulfonamide combination fish antibiotic offering broad-spectrum coverage against a range of bacterial infections in ornamental fish. Shop Fix Sulfa
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:
Contact Fine PetHealth.
Educational use: Our content is for informational purposes and is not a substitute for diagnosis or treatment from an aquatic or avian veterinarian.
I was worried about a couple of my fish, and decided I needed to treat with antibiotics ASAP. I picked a variety, just in case one didn’t work. Penicillin worked like a charm, and my fishes infections cleared up quickly. I owe Fine Pet a debt of gratitude because the antibiotics came quick and were of good quality/in date. As others have said, Fine Pet kept me updated the entire time so I knew exactly when my “cure” would be coming!
Thanks so much!
Hi nmtsaki, thank you so much for the incredibly detailed and kind review! We know exactly how stressful it is when your fish are sick and you need treatments ASAP. We are absolutely thrilled to hear the Fix Pen worked like a charm and that your aquarium is healthy again. Keeping you fully updated on shipping and providing fresh, high-quality products is our top priority. We truly appreciate your trust in us!
K
Kathy Carlson (Washington, IA)
Great price
High quality pet medication at a great price.
Hi Kathy, we really appreciate the perfect rating and the thumbs up on the Fix Pen! It’s great to know that you are pleased with both the quality and the price of the medication. Let us know whenever you need anything else!
T
T.M.
Easy to navigate website. Kept updated on shipping.
Hello Timothy! We are absolutely thrilled to receive your perfect rating. It is fantastic to know that you had a smooth shopping experience for your Fix Pen, from browsing our website to receiving those steady transit updates. We deeply appreciate your support and can't wait to see you back at our store whenever you need to restock!
M
Marion Mcbee (Concord, NC)
Fix Pen (Penicillin) Tablets Fish & Bird Antibiotic
J
Janice Waterman (Las Vegas, NV)
Fix Pen antibiotic
Great product. Arrived in just a few days, great condition and is doing the job.
