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Fish Pen (Penicillin) 500mg 30 Tablets - Fish Antibacterial for Flexibacter and Pop-eye
Fish Pen (Penicillin) 500mg 60 Tablets - Effective Treatment for Aquarium Fish
Fish Pen (Penicillin) 500mg 30 Tablets - Fish Antibacterial for Flexibacter and Pop-eye
Fish Pen (Penicillin) 500mg 60 Tablets - Effective Treatment for Aquarium Fish

Fish Pen (Fix Pen) Penicillin 500mg Fish Antibacterial

$69.99
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Penicillin for Fish and Pet Birds: Fin Rot, Pop-Eye and Skin Ulcers in Freshwater and Saltwater Tanks

Fish Pen (Fix Pen) is an antibacterial treatment for ornamental aquarium fish and non-food pet birds. Each tablet contains 500 mg of Penicillin V Potassium, made by Aqua Soma Labs in the United States, and aquarists use it for fin and tail rot, infected wounds, pop-eye and columnaris-type lesions where a bacterial cause is suspected, in both freshwater and saltwater tanks. The label amount is one 500 mg tablet per 20 gallons (76 L) of water, repeated every 24 hours for 5 to 10 days.

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 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.

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.

Important Notice: This product is intended for aquarium and ornamental fish and pet birds only. It is not for animals used for food.

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Penicillin V works on the cell walls of certain bacteria and can stop certain gram-positive bacteria from multiplying. Fix Pen is the fish penicillin 500 mg tablet many hobbyists use for fish tank infections, and the same tablets are used as penicillin for birds under avian veterinary guidance. It targets gram-positive and some gram-negative bacteria in ornamental aquarium use.

Fish-Specific Symptoms

  • Fin and tail rot (frayed or rotting fins)
  • Mouth rot and infected wounds
  • Pop-eye (bulging eyes) or cloudy eyes
  • Columnaris-type lesions (cotton-wool look)
  • Open sores, ulcers or reddening of the skin
  • Reduced appetite, lethargy or hiding

Bird-Specific Symptoms (Under Veterinary Guidance)

  • Respiratory infections with labored breathing
  • 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 treatment can fix. Penicillin does nothing for viral issues, and using fish treatments like penicillin when they are not needed can reduce how well they work later.

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.

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.

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 Penicillin Formula Fix Pen contains the same 500 mg Penicillin V Potassium per tablet that you used in Fish Pen and Fish Pen Forte. The formula 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.

  • Made in the USA with Fresh Dating

    Fix Pen is manufactured in the United States by Aqua Soma Labs and ships with fresh expiration dates, so you can keep a bottle on hand for your hospital tank and trust it when you need it.

  • 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 treatment.

  • Ideal for Hospital Tanks and Quarantine

    Treating in a separate hospital tank keeps the main aquarium's biofilter intact and limits exposure for healthy tank mates, so you can focus the treatment where it is needed.

Usage Guidelines & Instructions

Quick usage 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 stop if you see no improvement after 5 days.

Directions for Use:

  1. Set Up a Separate Treatment Area

    When possible, treat fish in a hospital or quarantine tank. This limits exposure for healthy tank mates and makes it easier to watch results and water quality. Remove activated carbon and turn off UV sterilizers during treatment, since they can strip the penicillin from the water.

  2. Directions 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 you are treating. Repeat every 24 hours, with a partial water change between treatments.

  3. Duration and When to Stop

    Continue for 5 to 10 days. If you see no improvement after 5 days, stop and consider asking an aquatic professional about next steps. Finish the full recommended number of days even if the fish look better, since stopping early can let the problem come back.

  4. Maintain Water Quality During Treatment

    Keep strong aeration and watch your 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.

  5. Pet Birds (Under Veterinary Guidance Only)

    For non-food pet birds, use penicillin only under avian veterinary guidance for amount, frequency and duration. Your vet can advise the correct amount for the bird's weight and decide whether to treat a single bird or the whole flock.

Fix Pen Amounts by Aquarium Size
Tank volume Penicillin per use 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. Continuing for the full recommended number of days matters as much as picking the right treatment. 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 treatment so the infection does not return. Afterward, run a partial water change and put activated carbon back in to clear any leftover penicillin.

Fix Pen treatment timeline A typical Fix Pen treatment for aquarium fish runs 5 to 10 days. Days 1 to 2: set up and start treatment. Days 3 to 5: watch for response. Day 5: reassess and stop if there is no improvement. Days 5 to 10: finish the treatment. Days 1 to 2 Set up & start Days 3 to 5 Watch for response Days 5 to 10 Finish the treatment Day 1 Day 5 Day 10 reassess; stop if no improvement
Fix Pen treatment timeline: a typical 5 to 10 day treatment for aquarium fish, with a reassessment point at day 5.

What You Need for Treatment

A little setup before the first dose saves trouble later.

  • A separate hospital or quarantine tank, so you are not treating 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 Fix Pen Works

Each Fix Pen tablet contains 500 mg of Penicillin V Potassium, a penicillin that acts on the cell walls of certain bacteria. It helps control certain gram-positive bacteria, and some gram-negative bacteria, when used as directed in ornamental fish care.

The rest of each tablet is made up of standard ingredients that hold it together on the shelf and let it break apart and dissolve when you add it to aquarium water or a treatment setup.

Safety and Tank Notes

When used as directed, Fix Pen usually causes no problems in ornamental fish. As with any treatment, unnecessary use, incorrect amounts or combining products can stress fish and reduce how well it works next time.

  • Fish

    Some fish may show short-term stress, reduced appetite or hiding 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, do 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 seems unwell, stop use and contact your avian veterinarian.

  • Invertebrates

    Do not use Fix Pen in display tanks that hold shrimp, snails or other invertebrates, since penicillin may harm them or disrupt the tank. 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 watch ammonia, nitrite and temperature closely. Do not mix Fix Pen with other treatments unless a veterinarian advises it.

  • Important safety note

    Fix Pen is intended for ornamental aquarium fish and non-food pet birds only. It is not for animals raised for food. Store tablets in a cool, dry place away from direct sunlight, and keep them 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.

More Fish & Bird Treatments

Fix Mox (Amoxicillin) 250/500mg A treatment 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, in ornamental fish. Available in 60-count bottles. Shop Fix Zole

Fix Doxy (Doxycycline) 100mg A treatment for columnaris, respiratory infections and Mycoplasma in fish and birds. Available in 30-count and 60-count tablets. Shop Fix Doxy

Fix Zithro (Azithromycin) 250mg A treatment 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 treatment 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 combination treatment offering broad coverage against a range of bacterial infections in ornamental fish. Shop Fix Sulfa

Fix Flex (Cephalexin) 250/500mg A treatment for fin rot, ulcers and bacterial skin infections in fish and birds. Shop Fix Flex

Fix Flox (Ciprofloxacin) 500mg A treatment for gram-negative bacterial infections, including fin rot, ulcers, columnaris and septicemia in ornamental fish. Shop Fix Flox

Browse all treatments Explore our complete Fish & Bird Antibacterials Collection for more Fix line products and related treatments.

Guides and treatment tips Visit the Fish & Bird Treatments Blog for practical usage guides, water-care advice and treatment tips.

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 usage and treatment decisions.

What is in each Fix Pen tablet?

Each Fix Pen tablet contains 500 mg Penicillin V Potassium. Penicillin V works by interfering with bacterial cell wall formation, which helps control certain gram-positive bacteria when used as directed.

What is the difference between Fish Pen Forte, Aqua Pen and Fix Pen?

Fish Pen, Fish Pen Forte, Aqua Pen and FISHBIOTIC Penicillin were Thomas Labs brand names for penicillin tablets sold for aquarium fish, and that line has been discontinued. Fix Pen from Aqua Soma Labs is the current replacement, with the same 500 mg Penicillin V Potassium per tablet in 30-count and 60-count bottles. The brand name and the maker changed; the tablet and its intended fish and bird use did not.

Is Fix Pen the same as Fin Pen?

Fin Pen is a separate brand of fish penicillin sold by other suppliers, not an earlier name for this product. Fix Pen is the Aqua Soma Labs version, at 500 mg of Penicillin V Potassium per tablet in 30-count and 60-count bottles. Both are penicillin tablets for aquarium fish, but strength and count differ between brands, so dose from the label on the bottle you actually have.

What is the fish penicillin dosage?

The label amount is one 500 mg tablet per 20 gallons (76 L) of water, roughly 25 mg per gallon, repeated every 24 hours with a partial water change between doses. Continue for 5 to 10 days and stop if there is no improvement after 5 days. For tanks under 20 gallons, such as a betta hospital container, splitting a tablet by hand is only approximate, so dissolve a whole tablet in a measured cup of tank water and add the matching share. Dose by the water volume of your treatment tank, not by the number of fish.

Is there a fish penicillin 250 mg version?

Fix Pen is sold only as a 500 mg tablet. Older hobby listings and other brands have offered penicillin for fish at 250 mg, which is why both strengths turn up in search results. If your plan calls for less than 500 mg, dissolve one Fix Pen tablet in a measured cup of tank water and add the matching share rather than looking for a 250 mg tablet in this line.

Can I use Fix Pen as penicillin for pet birds?

Fix Pen contains Penicillin V Potassium, the same penicillin used in avian care, so it can serve as a penicillin for birds option. Bird amounts are different from fish amounts and depend on the species and body weight, so use bird penicillin only under the direction of an avian veterinarian who can confirm a bacterial infection and set the amount and duration. Fix Pen is intended for non-food pet birds, not poultry raised for meat or eggs.

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. If your dog or cat needs penicillin, a licensed veterinarian should recommend the correct product and amount for that species.

Where can I buy penicillin for fish? Do Walmart or Tractor Supply sell it?

You can order Fix Pen, the current fish penicillin, directly from Fine Pet Health in 30-count and 60-count bottles of 500 mg tablets. General retailers such as Walmart, Tractor Supply, PetSmart and Petco do not carry the Aqua Soma Labs line, so a dedicated online supplier is the reliable route. In-stock orders ship from our U.S. warehouse via USPS or UPS, with same-day dispatch on orders placed before 1:30 p.m. CT, Monday through Friday.

Browse Fish & Bird Treatments Collection →
Can Fix Pen treat fungal, viral or parasitic infections?

No. Fix Pen is a penicillin that targets certain bacterial infections only. It does not work 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 the right treatment.

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Who is responsible for this page: the Fine PetHealth Editorial Team, part of Fine PetHealth Group LLC, a US-based pet supplies retailer.

How this page was created: prepared with AI assistance, verified against the current product label, and reviewed by a person before publishing. How we verify product information.

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