Before you treat: is this actually a bacterial infection?
Antibiotics only help true bacterial infections. A quick check first:
- Test your water first. Ammonia, nitrite, and nitrate problems cause most illness. Bad water is the #1 cause of fin rot, dropsy, and stress, and no antibiotic fixes water.
- Rule out parasites. White spots (ich) and a dusty velvet sheen are parasites; antibiotics do nothing for them.
- Use a hospital tank, never dose your main tank. Antibiotics kill the good bacteria in your filter, which spikes ammonia and can harm every fish.
- Confirm it is gram-positive. Penicillin is a narrow, gram-positive-focused drug. Many aquarium infections are gram-negative and need a different antibiotic, so a vet should confirm the cause before you reach for penicillin.
What penicillin is, and how it compares to amoxicillin
Fish Pen is the common name for fish penicillin, the penicillin sold for aquarium and fish-tank use. It comes in two strengths, fish penicillin 250 mg and fish penicillin 500 mg, the stronger version often listed as fish pen forte. Aquarium penicillin is a beta-lactam antibiotic that works mainly against gram-positive bacteria. People often confuse it with amoxicillin, and for good reason: amoxicillin is a semi-synthetic drug derived from penicillin, engineered for a broader reach.5 In practice, amoxicillin covers a somewhat wider range including more gram-negative bacteria, while penicillin stays more focused on gram-positive infections. Which one fits depends on the confirmed infection, which is a decision for a vet, not a guess. If you are weighing the two, our fish amoxicillin (Fish Mox) page covers that molecule in the same depth.
| Active ingredient | Penicillin V Potassium |
|---|---|
| Strength | 250 mg or 500 mg (Forte) per tablet |
| Also sold as | Various discontinued aquarium brand names (same drug) |
| Drug class | Penicillin (beta-lactam, gram-positive-focused) |
| Form | Tablets sealed in blister packs, open for tank dosing |
| Intended use | Ornamental / aquarium fish only |
| Availability | Prescription required; included with your order. Aquarium use only, not for human consumption. |
| Delivery to | All 50 US states, contact us for other countries |
Why blister packs, not a bottle
Each tablet is sealed in its own aluminum foil pocket, so opening one dose doesn’t expose the rest to air, light, and humidity. That matters for penicillin, which is moisture-sensitive.
A bottle is different. Every time you open it, the tablets still inside meet air, light, and humidity again. Sealed pockets also make tampering easy to see, and U.S. regulators count blister packs as tamper-evident. Blister packs cost us more than bulk bottles. The protection is worth it.
What it treats, and what it doesn’t
Where penicillin can help
Penicillin works mainly against gram-positive bacteria.1 Vets use it for the external infections those bacteria cause, such as fin and tail rot once a vet confirms a gram-positive cause.3 When a vet confirms a gram-positive infection, it can be an effective, targeted choice. The drug and dose remain the vet’s decision.
Where it’s the wrong choice
Penicillin doesn’t treat viral, fungal, or parasitic problems, and it’s weaker against the deep gram-negative infections behind many dropsy cases; those often need a fluoroquinolone or aminoglycoside.3 Columnaris (cotton-mouth, from Flavobacterium) and most septicemias are gram-negative and usually not a first-line penicillin case.2 Using it for the wrong problem wastes time a sick fish may not have. A vet should confirm the cause before you treat.
Take extra care with scaleless fish. Catfish, loaches, and similar species absorb medicine from the water much faster, so antibiotics hit them harder and can cause stress or toxicity.4 The rule aquatic vets use is “start low, go slow.” Penicillin can also harm invertebrates, so remove shrimp and snails before dosing, and it may affect your filter’s good bacteria — watch your water and treat in a hospital tank when you can.4 A fish vet should confirm the cause and the safe dose for your species before you treat.
Penicillin for fish: strengths and label rates by tank volume
Penicillin for fish is sold as Penicillin V Potassium in 250 mg and 500 mg (Forte) tablets. Older keepers knew the same product under discontinued brand names. The quick-reference below scales the manufacturer’s label rate to common tank sizes for planning only. It is not a prescription. Your fish veterinarian sets the actual amount, frequency, and duration, and takes extra care with scaleless species.
| Tank volume | Label rate | 500 mg tablets |
|---|---|---|
| 10 gallons (38 L) | ~250 mg | ½ |
| 20 gallons (76 L) | 500 mg | 1 |
| 40 gallons (151 L) | 1000 mg | 2 |
| 60 gallons (227 L) | 1500 mg | 3 |
| 75 gallons (284 L) | ~1875 mg | ~4 |
Dose by the water volume of your hospital or quarantine tank, not by the number of fish. Your aquatic veterinarian determines the correct amount for your tank volume and diagnosis, and always takes priority over this quick reference.
⚠ Important safety notes
- For aquarium fish only. Humans should not take it. Never take fish antibiotics yourself.
- Never dose your display tank. It can crash your biofilter and harm every fish — use a hospital tank.
- Take extra care with scaleless fish and invertebrates. Catfish, loaches, and scaleless species absorb medicine faster; remove shrimp and snails before dosing.
Cost & how to get it
The prescription is written as part of your order, with no separate consultation fee. The 250-count pack is the largest penicillin supply available for aquarium fish right now, in stock when gray-market sellers are sold out. The value here is the veterinary review and honest routing: if penicillin is not the right drug for your fish’s problem, we will tell you, and we won’t fill it.
| Pack size | 250 mg | 500 mg (Forte) |
|---|---|---|
| 30 tablets | $29.99 · $1.00/tab | $38.99 · $1.30/tab |
| 100 tablets MOST POPULAR | $54.99 · $0.55/tab | $71.99 · $0.72/tab |
| 250 tablets BEST VALUE | $119.99 · $0.48/tab | $155.99 · $0.62/tab |
How your order is reviewed and shipped
Place the order for the quantity you need. Instead of a public cart-and-pay step, our pharmacy emails you to confirm your delivery address and send a secure payment link. Nothing ships until that review is complete. It is how our pharmacy keeps eyes on every order and confirms it is intended for appropriate aquarium use. We email you at each step: once you place your order and send the form, our vet reviews it, a payment link goes out, your payment is confirmed, the order moves into processing, and a tracking link follows a few to several days later, so you never have to sit and refresh a page.
Related guides
Frequently asked questions about penicillin for fish
What does penicillin (Fish Pen) treat in fish?
Mainly gram-positive infections such as some fin and tail rot. Columnaris (cotton-mouth), dropsy, and most septicemias are gram-negative and often not a first-line penicillin case. A fish vet should confirm the cause and choose the drug and dose.
Is penicillin or amoxicillin better for fish?
Neither is simply better. Amoxicillin is derived from penicillin and covers a wider range; penicillin is more focused on gram-positive bacteria. The right choice depends on the confirmed infection — see our Fish Mox page and ask a vet.
Is penicillin safe for catfish, loaches, or scaleless fish?
Use extra caution. Scaleless fish absorb medicine faster and react more strongly, so vets advise “start low, go slow.” Confirm a safe dose for your species with a fish vet, and watch closely during treatment.
Will penicillin harm my shrimp, snails, or filter?
It can. Remove invertebrates like shrimp and snails before dosing, and know it may affect your filter’s beneficial bacteria. Treating in a hospital tank is safest.
Do I need a prescription to buy Fish Pen?
Yes — but you don’t have to arrange it yourself; a vet writes it as part of your order. Aquarium antibiotics require a veterinary prescription, and our licensed veterinarian issues it at no separate cost; you pay only for the medication.
Is penicillin for fish safe for humans?
No. This product is for ornamental aquarium fish only and isn’t for human use. If you need antibiotics for a person, see a doctor or pharmacist. You can read our guidance on this.
Medical disclaimer
This site is for informational purposes only and is not veterinary advice, diagnosis, or a treatment recommendation. A licensed veterinarian should determine medication selection, prescribing, dosing, and treatment for the individual animal. Never give a dog or cat antibiotics intended for aquarium fish or another species; the wrong drug or dose can seriously harm your pet. All medications sold on this site are for animal use only and are not intended for human use or consumption.
How we reviewed this page
This product page is prepared under veterinary oversight following our editorial standards. The clinical statements — what penicillin treats, the species-safety cautions, and where it is not the right choice — are checked against aquatic-veterinary references and reviewed and approved by our named fish veterinarian, Dr. Julia Bauer, before publication.
Sources & References
- Merck Veterinary Manual, "Bacterial Diseases of Fish." merckvetmanual.com (2024) Supports: gram-negative etiology of common fish infections; sensitivity-guided drug selection.
- MSU / SRAC Publication 478, "Aeromonas Bacterial Infections, Motile Aeromonad Septicemia." srac.msstate.edu (1998) Supports: aeromonads are gram-negative; exophthalmia and related signs.
- Merck Veterinary Manual, "Antibacterial Agents (Overview of Antimicrobial Therapy)." merckvetmanual.com (2024) Supports: penicillins are gram-positive-dominant beta-lactams; spectrum limits vs gram-negative infections.
- Fine Pet Health, "Penicillin: Handling Sensitive Species, Catfish, Loaches, and Scaleless Fish." finepethealth.com (2026) Supports: scaleless fish absorb medication faster (start-low-go-slow); invertebrate and filter cautions.
- Kim et al., "Comparative Pharmacological Assessment of Amoxicillin in Five Cultured Fish Species." NCBI PMC12024054 (2025) Supports: amoxicillin is a semi-synthetic antibiotic derived from natural penicillin with a broader spectrum.
