The original broad-spectrum antibiotic — a trusted first-line treatment for gram-positive bacterial infections in freshwater ornamental fish.

Why choose Penicillin VK? Penicillin V Potassium (Penicillin VK) is the original and most established antibiotic class in both human and veterinary medicine. It is particularly effective against gram-positive bacteria — the organisms most commonly responsible for mouth rot, popeye, and early-stage fin rot in freshwater fish. Its long track record, predictable activity profile, and gentle impact on the aquarium ecosystem make it an ideal first-line treatment before escalating to broader or more potent antibiotics. Also known as fish pen, it remains one of the most trusted aquarium antibiotics among experienced hobbyists.

About This Product

Penicillin V Potassium (Penicillin VK) is a broad-spectrum penicillin-class antibiotic effective against a wide range of gram-positive and gram-negative bacterial microorganisms. It works by inhibiting bacterial cell wall synthesis — preventing bacteria from building the protective wall they need to survive — causing them to rupture and die. As the foundational antibiotic class, it remains one of the most effective and well-understood treatments in aquarium medicine.

Each tablet contains USP-grade Penicillin V Potassium — available in 250mg and 500mg strengths, both in 100-tablet counts. Penicillin VK is an excellent first-line antibiotic for freshwater fish, particularly for gram-positive infections like mouth rot, popeye, and early fin rot. If treatment does not produce results within the first few days, consider stepping up to a cephalosporin (Cephalexin) or broader-spectrum antibiotic.

Also commonly searched as: fish penicillin, penicillin for fish, fish pen equivalent, penicillin VK fish antibiotic, aquarium penicillin, freshwater fish antibiotic penicillin, fish pen forte equivalent.

Commonly Treats

• Mouth rot (columnaris — early stage)
• Popeye (exophthalmia)
• Fin rot & tail rot
• Gill disease
• Gram-positive bacterial infections
• Bacterial hemorrhagic septicemia
• Skin infections & surface ulcers
• General freshwater bacterial disease

Directions for Use

1. Determine your dose: Use one 250mg tablet per 10 gallons of water, or one 500mg tablet per 20 gallons of water.
2. Dissolve the tablet in a small amount of aquarium water before adding to the tank. Stir until fully dissolved.
3. Add directly to your aquarium, pond, or hospital/quarantine tank.
4. Repeat every 24 hours for 5–10 days. Do not exceed 10 days of continuous treatment.
5. Perform a 25% water change each day before re-dosing to maintain water quality and reduce antibiotic buildup.

Product Specifications

• For ornamental fish use only. Not intended for use in fish raised for human consumption.
• Not for human consumption. Do not ingest. This product is sold for aquarium fish treatment only.
• Keep out of reach of children. Store in a cool, dry place away from children and pets.
• Do not exceed 10 days of continuous treatment. Overuse may contribute to antibiotic resistance in your aquarium ecosystem.
• Remove activated carbon from your filter before treatment — carbon will absorb the medication and render treatment ineffective.
• Primarily recommended for freshwater use. Penicillin VK can lose effectiveness in high-mineral or saltwater environments. For marine or hard water tanks, Minocycline or Doxycycline are more suitable alternatives.

Label & Usage Disclaimer: Please read and follow all label directions before use. Product descriptions and usage instructions shown here are taken directly from the manufacturer’s product label and information provided by Montana Aquarium Supply. These statements do not represent the views or recommendations of Pittsboro Pet Supply or Pittsboro Feed. Made in the USA. Distributed by Montana Aquarium Supply – Billings, MT.