Fish Penicillin 500mg (Fish Pen)

Description

Fish penicillin (Fish Pen) for aquarium fish

Fish penicillin, sold as Fish Pen (also called Fix Pen), contains penicillin, a penicillin antibiotic. It is sold under the Aqua Sea Mox brand. Its label lists it for common bacterial infections in ornamental freshwater and saltwater fish.

Available as: 500 mg, 30 tablets or 60 count.

Before you treat

• Test the water first: ammonia or nitrite above zero causes many of the same signs as an infection.
• Follow the dosing directions printed on the bottle label.
• Treat in a separate hospital tank where possible.
• Remove activated carbon from the filter during treatment and keep up regular water changes.
• Complete the full course, even if fish look better sooner.

For ornamental aquarium fish only. Use in pet birds only under a veterinarian’s direction. Not for human use and not for food-producing animals. Keep out of reach of children.

More fish antibiotics and antifungals

• Fish Mox (amoxicillin)
• Fish Mox Clavulanate (amoxicillin with clavulanate potassium)
• Fish Flex (cephalexin)
• Fish Doxy (doxycycline)
• Fish Minocycline (minocycline)
• Fish Flox (ciprofloxacin)
• Fish Levofloxacin (levofloxacin)
• Fish Zithro (azithromycin)
• Fish Zole (metronidazole)
• Fish Sulfa (sulfamethoxazole and trimethoprim)
• Fish Clin (clindamycin)
• Fish Ketoconazole (ketoconazole)
• Fish Fluconazole (fluconazole)

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Fish Pen questions

What is Fish Pen used for?

Fish Pen contains penicillin. Its label lists it for common bacterial infections in ornamental freshwater and saltwater aquarium fish. It may be used in pet birds only under a veterinarian’s direction.

How do I know my fish needs an antibiotic?

Test the water first (ammonia, nitrite, nitrate and temperature): many symptoms come from water quality, parasites or fungus rather than bacteria. The Merck Veterinary Manual notes that bacterial outbreaks in fish are often linked to poor water quality, handling or sudden temperature changes. Our guide How to choose an aquarium antibiotic explains the signs.

How do I use it?

Follow the dosing directions on the product label. Treat in a separate hospital tank where possible, remove activated carbon from the filter during treatment, and finish the full course.

Can I combine it with other medications?

Don’t combine different antibiotics or other medications in the same tank unless the product labels or an aquatic veterinarian say it’s safe.

Can people or food animals use it?

No. It is sold for ornamental aquarium fish only. It is not for human use and not for fish or animals raised for food. The FDA advises that animal drugs should not be used to treat people (Ornamental Fish Drugs and You).

Last reviewed: November 2026