Fish Pen (Fix Pen) 500mg 30/60ct

Fix Pen (Fish Pen Forte) Penicillin Tablets - 500 mg

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 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 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 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.

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.

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.

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 animals used for food.

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:

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.

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

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.

Shipping & Returns

Frequently Asked Questions

Bottom Line

Fix Pen is a fish penicillin 500 mg 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 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.

More Fish & Bird Treatments