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Fish Pen 250mg, 60 Tablets
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Retail: $44.99
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Thomas Labs Item #: 1017039 -

Penicillin Fish Antibiotics

Fish Pen and Fish Pen Forte are broad spectrum penicillin antibiotics for ornamental and aquarium fish. Fish Pen pet meds are effective against gram-positive and some gram-negative bacteria that cause a number of infections.

Fish Pen is a broad-spectrum antibiotic that effectively treats infected eyes, fin and tail rot, pop-eyes, columnaris disease, gill disease, skin ulcerations, and other diseases common to ornamental fish. The medication works quickly to improve symptoms and prevent a worsening of infection.

Benefits of Fish Pen:

  • Broad-spectrum action makes it effective against most bacterial infections in fish
  • Works against gram-positive and some gram-negative bacteria
  • Regular and extra strength tablets available over the counter
  • Works rapidly, usually within days
  • Generally well tolerated
  • Can be used to treat infections in dogs and cats
  • Once-daily dosing conveniently treats your entire tank

Use this medication only as directed by your veterinarian.

Do not give Fish Pen to fish or animals allergic to penicillin. Do not allow children to administer this medication.


  1. Add one 250 mg tablet to your tank for every 10 gallons of water. If instructed to do so by your veterinarian, use one 500 mg Fish Pen Forte tablet for every 10 gallons of water.
  2. Make a partial water change and then administer another dose 24 hours after then first.
  3. Continue dosing your tank once every 24 hours for 5 to 10 days.
  4. Stop using Fish Pen if your fish fail to improve after 5 days.
  5. Give cats and dogs 5 mg per pound of body weight by mouth every 8 hours.

Frequency of Use:

Use once daily for 5 to 10 days, unless instructed otherwise by your veterinarian.


Store Fish Pen at controlled room temperature, between 59 and 86 degrees F. Keep out of reach of children and pets.

It is a violation of Federal Law to use this product in a manner inconsistent with its labeling.

The penicillin in Fish Pen destroys bacteria by interfering with their ability to synthesize cell walls. Within minutes, the cell wall begins to weaken. Over time, the wall ruptures, effectively killing the bacteria.


Keep this product out of reach of children and animals. Do not use or store in or near open flame or heat.

Fish Pen is intended for use in ornamental fish only. May also be used in cats and dogs. Not for humans and not for use in fish intended for human consumption.

CAUTION: This medication does not usually cause side effects, but adverse reactions may occur when Fish Pen is used at high doses or for long periods.

Do not use this medication in animals with a known allergy to it or to similar medications.

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