Description
Fish Pen (Fix Pen) is fish penicillin: Penicillin V Potassium in 500 mg tablets from Aqua Soma Labs, for freshwater and saltwater aquarium fish. One dissolved tablet treats 20 gallons, repeated daily for 5 to 10 days according to the label.
- Active ingredient
- Penicillin V Potassium
- Strength
- 500 mg per tablet
- Bottle sizes
- 30 or 60 tablets
- Label dose
- 1 tablet per 20 gal (76 L)
- Course
- Once a day for 5 to 10 days; stop if no improvement after day 5
- Made by
- Aqua Soma Labs, USA
Where fish penicillin fits
Penicillin V is the narrowest antibacterial in the Fix line. It's aimed at gram-positive bacteria, and it does little against most gram-negative species. That narrow aim is the point: when the signs suggest a gram-positive infection, penicillin treats it without the broader disruption of a wide-spectrum antibacterial. Aquarists use Fish Pen for:
- Early fin rot, where the edges are just starting to fray
- Cuts, scrapes, and bite wounds that look like they're getting infected
- Popeye with erratic, spiraling swimming, a picture often linked to Streptococcus, a gram-positive bacterium
- Mild skin infections that follow handling or shipping stress
Fast-spreading ulcers, columnaris-type patches, and dropsy are usually gram-negative. Penicillin is a weak choice there; Fish Flox or Fish Sulfa Forte fit better. And like every antibacterial, it does nothing for ich, flukes, or fungus.
Fish Pen dosage by tank size
| Tank | Liters | 500 mg tablets per day |
|---|---|---|
| 10 gal | 38 L | ½ |
| 20 gal | 76 L | 1 |
| 30 gal | 114 L | 1½ |
| 40 gal | 151 L | 2 |
| 60 gal | 227 L | 3 |
| 80 gal | 303 L | 4 |
| 100 gal | 379 L | 5 |
Breaking a tablet by hand is approximate. For a tank under 20 gallons, dissolve one tablet in a measured volume of tank water, say 2 cups, and add the part that matches your tank: 1 cup for 10 gallons. Your bottle's label has the final word.
How to use Fish Pen
- Pick the treatment tank. A hospital tank is best. If you must treat the display, count its full water volume and remove invertebrates.
- Switch off what strips it out. Remove carbon and chemical media and turn off UV until the course is over.
- Dissolve each tablet first. Crush it in a cup of tank water and stir until it's gone, then pour it over the tank.
- Repeat daily with a water change. Replace part of the water, then add the full dose again.
- Check on day 5. No improvement means stop, per the label, and reconsider the diagnosis.
- Finish and recover. Keep going up to day 10 while the fish improve, then change a large share of the water and return the carbon.
How far a bottle goes
Because one tablet covers 20 gallons, Fish Pen goes further than its price suggests. A 10-day course in a 20 gallon hospital tank takes 10 tablets, about $11.70 at list price. A 40 gallon tank takes 20 tablets, about $23.30. The 60-count bottle is six full courses for a 20 gallon tank, so it's the one to keep if you quarantine new fish regularly.
Fish Pen and the broader options
| Product | Spectrum | Choose it for |
|---|---|---|
| Fish Pen | Narrow, gram-positive | Early fin rot, infected wounds, strep-type popeye |
| Fish Mox | Broader penicillin type | When you're not sure which bacteria are involved |
| Fish Flex | Cephalosporin | Ulcers and wounds that didn't respond to penicillins |
| Fish Cin | Gram-positive and anaerobes | Deep wounds and mouth rot |
Compare all nine on the fish antibacterials page.
Notes for the tank
- Even a narrow antibacterial can upset the filter's bacteria. Test ammonia and nitrite daily during treatment.
- Shrimp, snails, and corals don't belong in treated water.
- Penicillin breaks down in water over a day, which is why the label calls for a fresh dose every 24 hours.
- Don't mix Fish Pen with other treatments unless an aquatic veterinarian advises it.
- Keep the bottle closed, dry, and out of children's and pets' reach. Dispose of leftovers through a local take-back program, not the drain.
Fish Pen questions
Fish Pen, Fix Pen, Fish Pen Forte: what's what?
Fish Pen is the name aquarists search for penicillin tablets for fish. Fix Pen is the Aqua Soma Labs product. Forte usually means the 500 mg strength, and every Fix Pen tablet is 500 mg.
Is fish penicillin the same as amoxicillin?
They're relatives. Amoxicillin is a penicillin with a broader reach into gram-negative bacteria. Penicillin V stays narrow and gram-positive.
Is it safe for bettas?
Bettas are treated with the same label dose as other fish, measured on the water volume. In a 5 gallon tank that's a quarter tablet, so use the dissolve-and-measure method above.
Will it harm live plants?
Penicillin isn't aimed at plants, but if your plants live in the display tank, the simplest answer is to treat the fish in a separate hospital tank.
Where can I buy fish penicillin?
Many listings sell 250 mg tablets with a different dose rule. We stock Fix Pen 500 mg and ship from Missouri the same or next business day.
For aquarium and ornamental fish only.
