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Direct from Aqua Soma Labs with extended expiration dates.
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Suspected bacterial problems such as fin rot, tail rot, ulcers, infected wounds, and pop-eye
Form
Tablets, 500 mg each
Count sizes
30-count and 60-count bottles
Water type
Freshwater and saltwater
Made in
United States by Aqua Soma Labs
Fix Pen is an antibacterial treatment for ornamental aquarium fish and non-food pet birds.
Each tablet contains 500 mg of Penicillin V Potassium and is manufactured in the United States by Aqua Soma Labs.
Fix Pen is available in bottles containing 30 or 60 tablets with clearly printed expiration dates.
Aquarium keepers may know this product under older names such as Fish Pen, Fish Pen Forte, Aqua Pen, or FISHBIOTIC Penicillin.
These were names associated with discontinued Thomas Labs products, while Fix Pen is the current Aqua Soma Labs replacement.
Aquarium keepers may consider penicillin tablets when ornamental fish show suspected bacterial signs such as fin and tail rot, mouth rot, infected wounds, pop-eye, cloudy eyes, open sores, ulcers, reddened skin, or columnaris-type lesions with a cotton-wool appearance.
Penicillin V acts on the cell walls of certain susceptible bacteria and is generally associated with activity against Gram-positive bacteria and some Gram-negative organisms.
Fix Pen provides a consistent 500 mg tablet strength that can be dissolved in aquarium water or divided for a smaller treatment system.
The same tablets may be considered for non-food pet birds only under direct avian veterinary guidance.
Choose the 30-count bottle for a single hospital tank or occasional use, or the 60-count bottle for larger aquarium collections, multi-tank operations, or a veterinarian-directed aviary treatment plan.
Begin treatment at the first signs of a suspected bacterial concern. Symptoms differ between fish and birds, so review the separate lists below.
Fish-Specific Symptoms
Fin and tail rot with frayed or deteriorating fins
Mouth rot
Infected wounds
Pop-eye or bulging eyes
Cloudy eyes
Columnaris-type lesions with a cotton-wool appearance
Open sores, ulcers, or reddened skin
Reduced appetite, lethargy, or hiding
Bird-Specific Symptoms Under Veterinary Guidance
Respiratory signs with labored breathing
Soft-tissue concerns linked by a veterinarian to bacteria
Fluffed-up feathers
Lethargy or inactivity
Decreased appetite
Other symptoms specifically evaluated by an avian veterinarian
For birds, only use Fix Pen when an avian veterinarian has recommended penicillin for a specific condition and has determined the appropriate amount, administration method, frequency, and duration.
Stable water quality, appropriate nutrition, good housing, low stress, and careful observation remain essential parts of fish and bird care.
Penicillin does not treat viral problems, and unnecessary antibacterial use may reduce treatment effectiveness over time.
Same-day dispatch: In-stock orders placed before 1:30 p.m. CT, Monday through Friday, ship from our United States warehouse through USPS or UPS.
Important Notice: This product is intended for ornamental aquarium fish and non-food pet birds only. It is not intended for animals raised for food.
Fish Pen Forte vs Fix Pen: What Changed?
If you previously purchased Thomas Labs Fish Pen, Fish Pen Forte, Aqua Pen, or FISHBIOTIC Penicillin, here is what remains the same and what has changed.
Active ingredient
Same Penicillin Formula
Fix Pen contains Penicillin V Potassium at 500 mg per tablet, matching the familiar strength associated with Fish Pen and Fish Pen Forte.
Manufacturer
New Manufacturer
Fix Pen is manufactured in the United States by Aqua Soma Labs and replaces the discontinued Thomas Labs product line.
Tablet format
Same Dissolvable Tablet Form
The tablet can be dissolved in aquarium water or divided for smaller treatment volumes according to the current bottle label.
Bottle options
30-Count and 60-Count Bottles
Choose the bottle size that fits a single hospital tank, repeat treatment, multi-tank aquarium setup, or veterinarian-directed aviary plan.
Benefits of Fix Pen for Fish & Birds
01
Targeted Gram-Positive Bacterial Coverage
Penicillin is commonly considered when a susceptible Gram-positive bacterial component is suspected in fin damage, mouth rot, sores, ulcers, pop-eye, or cloudy eyes.
02
Made in the United States
Fix Pen is manufactured in the United States by Aqua Soma Labs and supplied with clearly printed strength and expiration information.
03
Consistent 500 mg Tablets
Each tablet provides a fixed 500 mg amount of Penicillin V Potassium, making treatment-water calculations easier across different aquarium volumes.
04
Common Aquarium Use Cases
Aquarium keepers may consider Fix Pen for suspected bacterial fin damage, infected wounds, flexibacter-type concerns, ulcers, and other symptoms where a susceptible bacterial cause is likely.
05
Freshwater and Saltwater Compatible
Fix Pen may be used in properly prepared freshwater or saltwater treatment tanks according to the current product label.
06
Quarantine-Friendly and Biofilter Aware
A separate hospital tank helps protect the display aquarium's beneficial bacteria, sensitive invertebrates, and healthy tank mates.
07
Vet-Directed Option for Pet Birds
Penicillin may be used for non-food pet birds only when an avian veterinarian has diagnosed the problem and established the appropriate treatment plan.
Usage Guidelines & Instructions
Quick Usage Summary for Fish
Use one 500 mg tablet per 20 gallons or approximately 76 litres of treatment water.
Repeat every 24 hours and perform a partial water change between treatments.
Continue for 5 to 10 days and discontinue use if no improvement is seen after five days.
1
Set Up a Separate Treatment Area
Treat affected fish in a hospital or quarantine tank whenever possible.
This limits exposure of healthy tank mates and makes it easier to monitor treatment response, ammonia, nitrite, temperature, and oxygen levels.
Remove activated carbon and other chemical filtration media and switch off UV sterilization during treatment.
2
Directions for Fish Aquariums
Dissolve one 500 mg tablet in a clean cup of aquarium water.
Add the dissolved mixture evenly throughout the treatment tank for every 20 gallons or approximately 76 litres of actual treatment water.
Calculate the real water volume after accounting for substrate, decorations, equipment, and fill level.
3
Repeat Every 24 Hours
Repeat treatment once every 24 hours according to the current bottle label.
Perform a partial water change between treatments to maintain suitable water quality.
4
Continue for 5 to 10 Days
Continue treatment for the complete recommended period, even when the fish begin to improve, unless a qualified professional advises otherwise.
Stopping treatment too early may allow the suspected bacterial problem to return.
5
Stop if There Is No Improvement
If no improvement is seen after five days, discontinue treatment and reassess the water quality and likely cause.
Seek aquatic veterinary or fish-health guidance if the fish become worse or develop severe distress.
6
Maintain Water Quality
Maintain strong aeration and stable temperature throughout treatment.
Test ammonia and nitrite regularly because antibacterial treatment may disturb beneficial filtration.
7
Pet Birds Under Veterinary Guidance Only
Do not use the aquarium-water chart to calculate an amount for birds.
An avian veterinarian must determine the correct amount according to species, weight, diagnosis, administration method, frequency, and duration.
Fix Pen Amounts by Aquarium Size
Tank volume
Penicillin per use
500 mg tablets
10 gallons (38 L)
250 mg
ยฝ tablet or the equivalent measured dissolved portion
20 gallons (76 L)
500 mg
1 tablet
40 gallons (151 L)
1,000 mg
2 tablets
60 gallons (227 L)
1,500 mg
3 tablets
75 gallons (284 L)
Approximately 1,875 mg
Approximately 4 tablets
These amounts follow the supplied label guideline of one 500 mg tablet per 20 gallons or approximately 76 litres, which equals about 25 mg per gallon. Dose according to the actual treatment-water volume, not the number of fish. For aquariums under 20 gallons, dissolving a complete tablet in a measured amount of water and adding the correct fraction may be more accurate than dividing the tablet by hand. The current bottle label or instructions from an aquatic veterinarian take priority over this quick-reference chart.
What to Expect During Treatment
Days 1 to 2
Fish may hide, eat less, or produce slightly cloudy treatment water. Maintain the treatment schedule and keep aeration strong.
Days 3 to 5
Look for sores and red areas that stop spreading, returning appetite, improved activity, and steadier breathing.
Days 5 to 10
Complete the recommended treatment. Afterward, perform a partial water change and restore activated carbon to help clear remaining treatment.
Reassess treatment at day five. If there is no improvement, or if the fish become worse, discontinue treatment and seek aquatic veterinary guidance.
What You Need for Treatment
A separate hospital or quarantine tank
A suitable aquarium heater
An air stone or established sponge filter
An ammonia and nitrite test kit
A suitable water conditioner or dechlorinator
A small net for gentle transfer
A clean cup for dissolving and measuring the treatment
Pro Tip: Dose according to the actual treated water volume rather than the aquarium's advertised capacity. Gravel, rocks, decorations, equipment, and the fill level reduce the amount of water inside the tank.
How Fix Pen Works
Each Fix Pen tablet contains 500 mg of Penicillin V Potassium.
Penicillin V works by interfering with the cell walls of certain susceptible bacteria.
It is generally associated with activity against certain Gram-positive bacteria and some Gram-negative organisms when used as directed in ornamental fish care.
The treatment is most appropriate when a susceptible bacterial cause is likely and should not be expected to treat fungal, viral, or parasitic conditions.
Each tablet also contains standard inactive ingredients that hold the tablet together during storage and allow it to break apart and disperse in aquarium water.
Use in non-food pet birds must follow the exact diagnosis and instructions of an avian veterinarian.
Safety and Tank Notes
When used according to the current label, Fix Pen usually causes no problems in ornamental aquarium fish.
Unnecessary use, inaccurate amounts, combining multiple treatments, or prolonged treatment may increase stress and antibacterial resistance.
Fish
Fish may temporarily hide, eat less, or appear mildly lethargic during treatment.
If severe distress develops, including rapid gill movement, surface gasping, loss of balance, rolling, or remaining on the bottom, stop treatment, increase aeration, and perform an appropriate partial water change.
Birds
Use Fix Pen in non-food pet birds only under avian veterinary guidance.
If a bird becomes unusually lethargic, stops eating, develops breathing difficulty, or appears significantly worse, discontinue use and contact the veterinarian.
Invertebrates
Do not use Fix Pen in display tanks containing shrimp, snails, corals, or other sensitive invertebrates whenever a separate treatment tank is available.
Penicillin may harm sensitive species or disturb the aquarium ecosystem.
Biofilter Awareness
Antibacterial treatment may affect beneficial filtration bacteria.
Monitor ammonia and nitrite closely and perform partial water changes whenever necessary.
Filtration During Treatment
Remove activated carbon and other chemical filtration media and switch off UV sterilization during treatment.
Restore normal filtration after treatment is complete.
Do Not Mix Treatments
Do not combine Fix Pen with other aquarium or bird treatments unless a qualified veterinarian specifically recommends the combination.
Important Safety Note
Fix Pen is intended for ornamental aquarium fish and non-food pet birds only.
It is not intended for animals raised for food.
Store tablets in their original, tightly closed container in a cool, dry place protected from direct sunlight, excessive heat, and moisture.
Keep out of reach of children and animals.
Always follow the directions printed on the current product label.
Shipping & Returns
Shipping Policy
Orders ship from our United States warehouse within one business day through USPS or UPS.
In-stock orders placed before 1:30 p.m. CT, Monday through Friday, may ship the same business day.
Shipping rates and estimated delivery times are calculated at checkout.
Fix Pen contains Penicillin V Potassium and is intended for certain suspected bacterial problems in ornamental aquarium fish, including fin and tail rot, mouth rot, infected wounds, ulcers, pop-eye, cloudy eyes, and some columnaris-type lesions.
Use in non-food pet birds should occur only under avian veterinary guidance.
What is the difference between Fish Pen Forte and Fix Pen?
Fix Pen is the current Aqua Soma Labs replacement for discontinued Thomas Labs Fish Pen and Fish Pen Forte products. It provides the same 500 mg Penicillin V Potassium tablet strength with updated packaging.
Is Fix Pen a replacement for FISHBIOTIC Penicillin or Aqua Pen?
Fix Pen is presented as the current replacement for older FISHBIOTIC Penicillin and Aqua Pen aquarium products.
How soon should I see improvement with Fix Pen?
Improvement may appear within three to five days through sores that stop spreading, improved appetite, increased activity, and steadier breathing.
Discontinue treatment and seek professional advice if the fish worsen or no improvement appears by day five.
Where can I buy penicillin tablets for fish online?
Fix Pen 500 mg tablets can be ordered through this online store in 30-count or 60-count bottles and shipped according to the current store shipping policy.
What is in each Fix Pen tablet?
Each tablet contains 500 mg of Penicillin V Potassium together with standard inactive tablet ingredients.
What is the Fix Pen amount for fish?
The supplied label guideline is one 500 mg tablet per 20 gallons or approximately 76 litres of treatment water.
How long should I treat with Fix Pen?
The supplied instructions describe treatment every 24 hours for 5 to 10 days, with a partial water change between treatments.
Discontinue treatment if no improvement is seen after five days.
Should I treat in the main tank or a hospital tank?
A separate hospital or quarantine tank is strongly preferred. This protects healthy tank mates, sensitive invertebrates, and the display aquarium's established filtration.
Do I need to remove activated carbon or turn off UV?
Remove activated carbon and other chemical filtration media and switch off UV sterilization during treatment because these systems may reduce treatment availability in the water.
Will Fix Pen harm beneficial aquarium bacteria?
Antibacterial treatment may disturb beneficial biological filtration. Use a separate treatment tank whenever possible and monitor ammonia and nitrite closely.
Can Fix Pen be used in saltwater aquariums?
Fix Pen is presented for properly prepared freshwater and saltwater treatment tanks. Use a separate hospital tank when the display system contains corals or sensitive invertebrates.
Is Fix Pen safe for betta fish and small aquariums?
Small aquariums require careful measurement. The supplied rate is one 500 mg tablet per 20 gallons, so a smaller treatment volume requires a proportional amount.
Dissolving one tablet in a measured volume and adding the appropriate fraction may be more accurate than dividing the tablet by hand.
Can I use Fix Pen in a planted tank or with shrimp and snails?
Treat affected fish in a separate hospital tank whenever possible. Penicillin may disturb the display aquarium and may harm shrimp, snails, or other sensitive invertebrates.
Can Fix Pen treat fungal, viral, or parasitic infections?
Fix Pen is an antibacterial product and should not be expected to treat fungal, viral, or parasitic conditions.
Can I combine Fix Pen with other fish treatments?
Do not combine Fix Pen with other aquarium treatments unless a qualified veterinarian specifically recommends the combination.
What should I do after finishing a Fix Pen treatment?
Perform a partial water change, resume normal filtration, restore activated carbon if normally used, and continue monitoring ammonia, nitrite, temperature, and fish behavior.
Can I use Fix Pen for dogs, cats, or chickens?
No. This product is labeled for ornamental aquarium fish and non-food pet birds. It is not intended for dogs, cats, humans, chickens raised for food, or other food-producing animals.
Can I use Fix Pen as penicillin for pet birds?
Fix Pen may be considered for non-food pet birds only under direct avian veterinary guidance. The veterinarian must establish the diagnosis, amount, administration method, frequency, and duration.
How should I store and dispose of Fix Pen tablets?
Store tablets tightly closed in their original container in a cool, dry location protected from direct sunlight, excessive heat, and moisture.
Follow the current product label and applicable local requirements when disposing of unused tablets, treatment water, or filter media. Avoid releasing concentrated treatment into natural waterways.
Bottom Line
Fix Pen is a 500 mg Penicillin V Potassium tablet for certain suspected bacterial problems in ornamental aquarium fish and a veterinarian-directed penicillin option for non-food pet birds.
It is the current Aqua Soma Labs replacement for discontinued Thomas Labs Fish Pen and Fish Pen Forte products.
The supplied aquarium guideline is one 500 mg tablet per 20 gallons or approximately 76 litres of treatment water, repeated every 24 hours for 5 to 10 days with partial water changes between treatments.
Reassess treatment at day five and discontinue use if no improvement is seen.
Fix Pen will not treat fungal, viral, or parasitic conditions and is not intended for animals raised for food.
Use a hospital tank whenever possible and consult an aquatic or avian veterinarian when the diagnosis is uncertain or when treating a bird.
More Fish & Bird Treatments
Fix Mox (Amoxicillin) 250/500 mg
A treatment for fin rot, dropsy, ulcers, infected wounds, and other suspected bacterial concerns in ornamental fish.
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Transparency & Verification
How Product Information Is Verified
Who is responsible for this page: The store editorial team responsible for reviewing and maintaining product information.
Verification snapshot: This page is written from the current product label for this item and reviewed for accuracy and consistency before publishing.
How We Verify Product Information
Label-First Accuracy
Directions, treatment duration, storage information, and warnings are taken from the current product label.
Specification Validation
The active ingredient, tablet strength, bottle counts, and manufacturer information are cross-checked against available product specifications.
Consistency Checks
Product content is reviewed for consistent strength information, directions, storage details, and safety language throughout the page.
Updates
Pages are updated when product labels, packaging, tablet counts, manufacturer details, or directions change.
What we do not do: We do not invent product claims, and we do not add usage directions beyond what the current product label states for its intended use.
See something that does not match the product you received? Please contact us so the information can be reviewed and updated.
Educational use: This content is provided for general informational purposes only and is not a substitute for advice from a qualified veterinarian.
