Fix Flex is a pharmaceutical-grade, broad-spectrum fish and bird antibiotic in capsule form for ornamental fish and pet birds. Manufactured by Aqua Soma Labs, it supplies cephalexin monohydrate in 250 mg and 500 mg capsule strengths and is available in bottles of 30 or 100 capsules with fresh expiration dates.
Often called Fish Flex or Fin Flex in the hobby community, this product serves as a reliable online alternative to discontinued Thomas Labs Fish Flex, Fish Flex Forte, Aqua Ceph, Aqua Flex and FISHBIOTIC Cephalexin products. Aquarists and bird keepers also search for it under names like "fish keflex" or "cephalexin for fish." It helps manage common bacterial issues such as fin and tail rot, skin ulcers, infected wounds and lesions, red patches, pop-eye, and dropsy (pinecone disease) in freshwater and saltwater aquariums. Bird keepers also use it, under veterinary guidance, for conditions like bumblefoot, skin infections or joint infections in pigeons and other non-food pet birds.
Cephalexin is a first-generation cephalosporin antibiotic that works by disrupting bacterial cell wall formation, making it especially active against susceptible Gram-positive bacteria and some Gram-negative species. The capsules can be opened so the powder dissolves quickly and the dose spreads evenly through the water or a separate hospital tank.
This antibiotic from Aqua Soma Labs gives hobbyists flexible options with two strengths and two bottle sizes, whether they maintain a single hospital tank, multiple community aquariums, or need treatment support for a small bird flock or aviary.
Use this product when you notice early signs of bacterial stress such as frayed or ragged fins, red or pale fin edges, open sores, localized swelling, cloudy patches on the body, pop-eye (exophthalmia) or a persistent loss of appetite in fish. For birds, watch for swollen foot pads (bumblefoot), skin redness, joint swelling, ruffled feathers or reduced activity. Addressing potential bacterial issues early with appropriate treatment, together with good water quality, suitable housing and advice from an aquatic or avian professional when available, helps your fish and birds maintain a stable, healthy aquarium or aviary.
Fix Flex is not a substitute for human Keflex or any human prescription medication. It is formulated, labeled and sold exclusively as an antibiotic for ornamental aquariums and pet birds.
Order online and enjoy same-day dispatch on orders placed before 1:30 p.m. CT, Monday through Friday. Your order ships directly from our U.S. warehouse via USPS or UPS.
Important Notice: This product is intended for aquarium and ornamental fish and pet birds only. It is not for human consumption or for animals used for food. We do not sell or recommend this product for human use.
Fish Flex vs Fix Flex: What Changed?
If you previously purchased Thomas Labs Fish Flex, Fish Flex Forte, Aqua Ceph, Aqua Flex or FISHBIOTIC Cephalexin, here is what has changed and what remains the same:
Same Active Ingredient The replacement contains the same cephalexin monohydrate per capsule that you relied on with Fish Flex and Aqua Ceph. The pharmaceutical-grade formulation targets the same bacteria.
New Manufacturer The new product is made by Aqua Soma Labs, replacing the discontinued Thomas Labs line. The company manufactures in the United States under strict quality-control standards.
Same Capsule Form It uses the same capsule format as the original Fish Flex, making it easy to open and dissolve the powder in aquarium water or mix with soft food for birds.
Available in Two Strengths and Two Sizes Choose 250 mg or 500 mg capsules in bottles of 30 or 100, matching the flexibility that Fish Flex and Fish Flex Forte offered for different aquarium volumes and treatment plans.
Benefits of Fix Flex for Fish & Birds
Broad-Spectrum Bacterial Support
This product provides broad-spectrum cephalexin coverage for aquarium fish and pet birds. It helps manage common problems such as fin and tail rot, skin ulcers, infected wounds, red patches and early signs of bacterial stress when used as directed.
No Veterinary Prescription Required
It is available over the counter for aquarium and avian use, so you can start treatment promptly without waiting for a prescription. For birds, professional veterinary guidance is still strongly recommended to determine the correct dose and duration.
Two Strengths for Flexible Dosing
Use 250 mg capsules for smaller aquariums, hospital tanks or lightly stocked systems, and 500 mg capsules for larger volumes or when higher total doses are needed, always following the label directions.
Consistent Pharmaceutical-Grade Capsules
Each capsule delivers a reliable dose of the active ingredient, making it easier to scale treatment from a single quarantine tank up to larger community aquariums, lofts or aviaries without complicated calculations.
Easy to Administer
These capsules can be opened and the powder dissolved directly in aquarium water or mixed with soft food, providing flexible options for shy, stressed or hard-to-handle fish and birds while minimizing handling and extra stress.
Versatile for Freshwater, Saltwater & Aviary Setups
Hobbyists reach for this antibiotic as their trusted choice in both freshwater and marine systems, and as a convenient option for pigeons and other ornamental birds kept in lofts, cages or aviaries when this class of antibiotic is appropriate.
Ideal for Hospital Tanks and Quarantine
Using this antibiotic in a separate hospital tank or treatment cage helps protect the main aquarium's biofilter and reduces unnecessary exposure of healthy tank mates or flock members while you focus treatment where it is needed most.
Dosage Guidelines & Instructions
Directions for Use:
Set Up a Separate Treatment Area
Move affected fish to a dedicated hospital tank and, where practical, house birds needing treatment in a small treatment cage. This helps protect healthy tank mates or flock members and lets you control dosing more precisely.
Choose the Right Strength
These cephalexin capsules are available in 250 mg and 500 mg strengths. Select the strength that matches your aquarium volume or your professional's advice, and follow the product label for the number of capsules and how often to dose. Do not increase the dose or shorten the intervals on your own.
Dose for Fish Aquariums
Open the capsule and dissolve the powder in a small cup of aquarium water, then distribute it evenly around the hospital tank. Follow the label directions for the correct number of capsules per gallon and dosing frequency. Repeat the dose as directed for the full recommended course while closely monitoring water quality and fish behavior.
Use in Pet Birds (Under Veterinary Guidance)
Only administer this medication to pigeons and other pet birds under the direction of an avian veterinarian. Your vet can advise you on the correct mg-per-kg dose, frequency and duration. Capsule contents may be mixed into soft food or drinking water exactly as instructed.
Maintain Water Quality & Complete the Course
Perform partial water changes as needed between doses to keep ammonia and nitrite low and ensure good aeration in the hospital tank. Continue treatment for the full recommended course even if fish or birds appear better; stopping early can allow symptoms to return. If there is no improvement after several days, or if signs worsen, consult an aquatic or avian professional.
Pro Tip: Always quarantine infected fish and birds away from healthy companions during treatment to reduce stress and help prevent the spread of disease. A small, dedicated hospital tank with a heater, air stone and no substrate makes dosing and water changes easier.
How This Medication Works
Active ingredient: Cephalexin Monohydrate (250 mg or 500 mg per capsule)
Cephalexin is a first-generation cephalosporin antibiotic that disrupts bacterial cell wall formation, helping slow or stop the growth of susceptible Gram-positive bacteria and some Gram-negative species in the skin, fins and soft tissue of fish and birds.
Inactive ingredients
Hard gelatin capsule with standard pharmaceutical excipients selected to protect the active powder, support shelf life and help the contents disperse evenly when dissolved in aquarium water or mixed with soft food.
Side Effects & Safety Tips
When used as directed, this product is generally well tolerated in ornamental fish and pet birds. As with any antibiotic of this class, incorrect dosing or prolonged use can increase the risk of side effects and antibiotic resistance.
Fish
Antibiotics of this type may cause temporary stress or hiding behavior, mild water cloudiness and reduced appetite during treatment. If fish show severe distress such as rapid gill movement, loss of balance, gasping at the surface or lying on the bottom, stop treatment, improve aeration and seek aquatic veterinary advice.
Birds
When the medication is given in food or drinking water, possible reactions include decreased appetite, loose or changed droppings and lethargy, especially if the dose is too high or the bird is sensitive. If your bird appears fluffed up, inactive or stops eating, discontinue use and contact an avian veterinarian.
Invertebrates
Do not use this product in display tanks that contain shrimp, snails or other invertebrates, as strong antibiotics may harm them or disrupt the tank ecosystem. Whenever possible, move affected fish to a separate hospital tank.
Safe use in your system
Treat fish in a hospital or quarantine tank whenever you can, remove activated carbon and turn off UV sterilizers during treatment, and monitor ammonia, nitrite and temperature closely. Do not mix this medication with other antibiotics unless advised by a veterinarian.
Important safety warning
Fix Flex is intended for ornamental aquarium fish and non-food pet birds only. It is not for human use and not for animals raised for food. Using fish or bird antibiotics in people instead of prescribed medicine can lead to treatment failure, serious side effects and antibiotic resistance. Store capsules in a cool, dry place away from direct sunlight and keep out of reach of children and pets. Always follow the directions on the label.
Shipping & Returns
Shipping Policy: Orders ship from our U.S. warehouse within 1 business day via USPS or UPS, with same-day dispatch on orders placed before 1:30 p.m. CT (Monday through Friday). For full shipping details, please see our shipping policy.
Refund Policy: If there is an issue with your order, you may be able to return or exchange items within 30 days according to our store guidelines. Before sending anything back, please review our refund policy carefully.
Frequently Asked Questions
What is Fix Flex (Fish Flex) used for in fish and birds?
Fix Flex (cephalexin 250/500 mg) is an antibiotic used by hobbyists to help manage bacterial issues such as fin and tail rot, skin ulcers, infected wounds, red patches and pop-eye in ornamental aquarium fish, and to support veterinarian-directed treatment of conditions like bumblefoot, skin infections or joint infections in pet birds.
What is the difference between Fish Flex and Fix Flex?
Fish Flex was the brand name used by Thomas Labs for their cephalexin capsules. After Thomas Labs discontinued the product, Aqua Soma Labs introduced Fix Flex as a direct replacement. It contains the same cephalexin monohydrate, uses the same capsule format, and is intended for the same aquarium fish and pet bird applications.
Is Fix Flex a replacement for Fish Flex Forte?
Yes. If you previously used Fish Flex Forte 500 mg, the replacement from Aqua Soma Labs offers the same active ingredient (cephalexin monohydrate) and is available in both 250 mg and 500 mg capsule strengths in 30-count and 100-count bottles.
Is Fix Flex the same as Aqua Ceph or Aqua Flex?
Aqua Ceph and Aqua Flex were other names used for cephalexin fish antibiotic products. The replacement from Aqua Soma Labs contains the same cephalexin monohydrate active ingredient and serves as a modern alternative to older Aqua Ceph and Aqua Flex products that are no longer manufactured.
Is Fix Flex a human Keflex alternative?
No. Fix Flex is not a human Keflex alternative. While it contains cephalexin, this product is strictly formulated, labeled and sold as an aquatic and avian antibiotic for aquariums and pet birds. It must never be taken by humans or given to animals raised for food. It is never a substitute for human medicine.
Do I need a veterinary prescription to buy Fix Flex?
No. It is sold over the counter as an aquatic and avian antibiotic, and no veterinary prescription is required to purchase it. However, for pet birds, professional veterinary guidance is strongly recommended to determine the correct mg-per-kg dose and treatment duration.
Where can I buy cephalexin for fish online?
You can buy this product (cephalexin 250/500 mg) directly from Fine Pet Health. Orders ship from our U.S. warehouse with same-day dispatch on orders placed before 1:30 p.m. CT, Monday through Friday, via USPS or UPS.
How do I know if my fish might need a cephalexin antibiotic?
Watch for signs such as frayed or rotting fins, red or white ulcers, open sores, localized swelling, pop-eye or listless behavior that does not improve with good water quality and husbandry. A fish veterinarian or experienced aquarist can help confirm whether a bacterial infection is likely and whether antibiotic treatment is appropriate.
How do I choose between the 250 mg and 500 mg capsules?
These capsules are available in 250 mg and 500 mg strengths to suit different aquarium volumes and treatment plans. Use the 250 mg capsule for smaller hospital tanks or lightly stocked systems, and the 500 mg capsule for larger volumes. Always follow the product label or your aquatic or avian professional's advice for the correct number of capsules and dosing frequency.
Can I use Fix Flex in my main community or reef tank?
It is best to treat sick fish in a separate hospital tank instead of the main aquarium. This antibiotic and similar products are not recommended for reef tanks or setups with shrimp, snails, corals or other invertebrates, because strong antibiotics may harm them or disrupt beneficial filter bacteria.
Does Fix Flex work in saltwater aquariums?
Yes. It can be used in both freshwater and saltwater hospital tanks. Dissolve the capsule contents following the label directions and maintain stable salinity, temperature and water quality parameters throughout the treatment course.
How long should I treat with Fix Flex?
Follow the dosage and duration on the product label or as advised by an aquatic veterinarian. Birds should only receive this medication for the dose and duration prescribed by an avian vet. Always complete the full recommended course even if fish or birds appear better; stopping early can allow symptoms to return.
Can Fix Flex be used for pet birds?
This antibiotic can be used in pigeons and other pet birds only under the direction of an avian veterinarian, who will calculate the mg per kg dose and decide whether to treat a single bird or an entire flock. Capsule contents may be mixed into soft food or drinking water exactly as instructed. Never guess bird dosages without professional guidance.
Can I use Fix Flex for my dog or cat?
No. Fix Flex is for aquarium fish and non-food pet birds only. It must not be used for people, dogs, cats or other mammals. If you suspect an infection in yourself or another animal, contact a medical or veterinary professional instead of using fish antibiotics.
What is the active ingredient in Fish Flex?
The active ingredient in Fish Flex (now Fix Flex) is cephalexin monohydrate, a first-generation cephalosporin antibiotic. These capsules are available in 250 mg and 500 mg strengths and are formulated for ornamental fish and pet birds.
Will Fix Flex harm beneficial aquarium bacteria?
Any broad-spectrum fish antibiotic, including cephalexin for fish, can affect beneficial bacteria if used in the main tank. Using a hospital tank, monitoring ammonia and nitrite, and performing partial water changes helps protect the biofilter while treatment is underway.
Can I combine Fix Flex with other fish medications?
Combining antibiotics or mixing this product with antifungal or antiparasitic medications is generally not recommended unless specifically advised by an aquatic veterinarian. Using multiple medications at once can stress fish, cause unpredictable interactions and make it harder to identify which treatment is working.
How quickly should I see improvement?
Many aquarists notice clearer skin, improved fin condition and more normal swimming or appetite within a few days once water quality is stable and the full course is followed. If fish or birds fail to improve or seem worse after several days, stop treatment and seek aquatic or avian veterinary advice.
Can Fix Flex treat fungal, viral or parasitic infections?
No. This product is a cephalexin-based antibiotic that targets certain bacterial infections only. It is not effective against fungi, viruses or parasites such as ich, flukes or worms. For fungal, parasitic or viral problems, work with an aquatic or avian veterinarian to choose an appropriate treatment instead of using an antibiotic.
How should I store and dispose of Fix Flex capsules?
Store capsules in their original container in a cool, dry place away from heat, moisture, children and pets. Follow local guidelines for pharmaceutical disposal and avoid flushing unused capsules down the drain unless specifically instructed by local authorities.
More Fish & Bird Antibiotics
Fix Mox (Amoxicillin) 250/500mg A broad-spectrum penicillin-type fish antibiotic for fin rot, dropsy, ulcers and other bacterial infections. Available in 30-count and 100-count capsules. Shop Fix Mox
Fix Zole (Metronidazole) 250/500mg Targets anaerobic bacteria and protozoa including Hexamita and hole-in-the-head disease in ornamental fish. Available in 60-count bottles. Shop Fix Zole
Fix Doxy (Doxycycline) 100mg A tetracycline-class fish and bird antibiotic effective against columnaris, respiratory infections and Mycoplasma. Available in 30-count and 60-count tablets. Shop Fix Doxy
Fix Zithro (Azithromycin) 250mg A macrolide fish and bird antibiotic for fin rot, columnaris, gill disease and avian respiratory concerns. Available in 12-count and 30-count tablets. Shop Fix Zithro
Fix Cin (Clindamycin) 150mg A lincosamide fish antibiotic commonly used for gram-positive bacterial infections including skin and soft tissue concerns in ornamental fish. Shop Fix Cin
Fix Sulfa (SMZ/TMP) 960mg A sulfonamide combination fish antibiotic offering broad-spectrum coverage against a range of bacterial infections in ornamental fish. Shop Fix Sulfa
Fix Pen (Penicillin) 500mg A penicillin-type fish and bird antibiotic for gram-positive bacterial infections in ornamental fish and non-food pet birds. Shop Fix Pen
Fix Flox (Ciprofloxacin) 500mg A fluoroquinolone fish and bird antibiotic for gram-negative bacterial infections including fin rot, ulcers, columnaris and septicemia in ornamental fish. Shop Fix Flox
Guides and treatment tips Visit the Fish & Bird Antibiotics Blog for practical dosing guides, water-care advice and treatment tips.
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About Fine PetHealth and How We Verify Our Antibiotic Guides
Who is responsible for this page: Fine PetHealth Editorial Team.
Verification snapshot: This guide is written from the current product label for this item and reviewed for accuracy and consistency before publishing.
How we verify product information (our checklist):
1) Label-first accuracy: Dosing, duration, storage, and warnings are taken from the current product label for this item.
2) Ingredient validation: Active ingredient names are cross-checked against reputable references (for example, NIH PubChem) to reduce the risk of labeling mistakes.
3) Consistency checks: We review for internal consistency (the same dosing and storage language across sections) and remove wording that could overpromise results.
4) Updates: We update pages when label directions, packaging, or manufacturer information changes.
What we don’t do: We don’t invent diagnoses, and we don’t create dosing instructions beyond what the product label states for this item’s intended use.
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Educational use: Our content is for informational purposes and is not a substitute for diagnosis or treatment from an aquatic or avian veterinarian.
This is a very reliable company and I’m satisfied with the products I received.
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Nel
Thank you❤️ product is awesome..
Hi! Thank you so much for the lovely review and the sweet emoji. We are incredibly happy to know that you are finding your Fix Flex purchase to be awesome. Your positive feedback means everything to our team, and we are eagerly looking forward to serving you again in the future!
