Aquarium fish antibiotics
Cephalexin for Fish: Fish Flex Capsules (250 & 500 mg)
A broad-spectrum cephalosporin for ornamental fish. Intended for aquarium fish only, not for human use.
These images are for illustrative purposes only.
Delivery in 10–14+ business days. Restock your aquarium’s medicine shelf before your fish need it. This is for planning ahead, not same-day treatment.
No public checkout. After you place an order, our pharmacy confirms your address and sends a secure payment link. A qualified team member reviews every order for aquarium use before it ships. Your card isn’t charged until you approve the link.
New here? See how our free vet prescription works. You pay only for the medication; a licensed vet must approve it first. A licensed vet reviews every order, with no separate consult fee.
Quick answer: Cephalexin for fish, sold as Fish Flex (or Fish Keflex), is a broad-spectrum cephalosporin antibiotic. It treats bacterial infections a vet has identified, fin rot, skin ulcers, gill disease, and columnaris. A fish vet should confirm the cause; drug choice and dose are the vet’s call. For aquarium fish only; not for human use.
Not sure of your tank size? Use our free aquarium tank volume calculator to get the true water volume for accurate dosing.
What fish flex is
Fish Flex is the common name for cephalexin sold for aquarium fish (you may also see it called Fish Keflex). Cephalexin is a cephalosporin. A broad-spectrum antibiotic that works well against many gram-positive bacteria and some gram-negative ones. This product is the same molecule, manufactured and labeled specifically for ornamental fish.
We are a licensed, vet-staffed pharmacy, not a repackager. Our job is to sell you the right treatment, and to help you use cephalexin for the right reason. A vet confirms the cause and the dose. That care is the whole point.
| Active ingredient | Cephalexin (monohydrate) |
|---|---|
| Strength | 250 mg or 500 mg (Forte) per capsule |
| Count | 30 or 100 capsules |
| Drug class | Cephalosporin (broad-spectrum) |
| Form | Capsules sealed in blister packs, open for tank dosing |
| Intended use | Ornamental / aquarium fish only |
| Availability | Over-the-counter for aquarium use only, not for human consumption |
| Delivery to | Free to all 50 US states & Canada, other countries, contact us |
Why blister packs, not a bottle
Each capsule is sealed in its own aluminum foil pocket, backed by foil on both sides. That keeps every dose away from moisture and light until you use it. This matters for cephalexin, which is sensitive to moisture.
A bottle is different. Every time you open it, the capsules 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 cephalexin can help
Cephalexin is a broad-spectrum cephalosporin.1 Vets reach for it for external bacterial infections a fish shows on the outside, fin and tail rot, skin ulcers and wounds, gill disease, and columnaris.2 It is strongest against gram-positive bacteria and covers some gram-negative ones. When a vet confirms an infection it covers, it can be an effective, gentle choice. The drug and dose remain the vet’s decision.
Where it’s the wrong choice
Cephalexin doesn’t treat viral, fungal, or parasitic problems, and it’s not the first pick for the deep gram-negative infections behind many dropsy cases, those often need a fluoroquinolone or aminoglycoside.3 Overusing any antibiotic drives resistance, so it should be used only when a vet confirms it’s needed and, where possible, after testing.
Take care with your tank: remove activated carbon before dosing, treat in a hospital tank when you can, and monitor water quality. And a fish vet should confirm the cause before you treat.
Fish flex strengths and label rates by tank volume
Fish flex is sold as cephalexin (cephalexin monohydrate) capsules in a 250 mg option and a 500 mg “Forte” option. The same product older keepers knew as Fish Flex, Fish Flex Forte, or Fix Flex (Thomas Labs brand names). The quick-reference below scales a common manufacturer’s label rate of about 250 mg per 10 gallons to typical tank sizes. It is the label rate for planning, not a prescription. Your fish veterinarian sets the actual dose, frequency, and duration.
| Tank volume | Label rate (cephalexin) | 250 mg capsules | 500 mg capsules |
|---|---|---|---|
| 10 gallons (38 L) | 250 mg | 1 | — |
| 20 gallons (76 L) | 500 mg | 2 | 1 |
| 40 gallons (151 L) | 1000 mg | 4 | 2 |
| 60 gallons (227 L) | 1500 mg | 6 | 3 |
Dose by the water volume of your hospital or quarantine tank, not by the number of fish, and pre-dissolve the capsule contents. The bottle label and your aquatic veterinarian always take priority over this quick reference. Cephalexin leans toward skin and soft-tissue (gram-positive) infections; for the gram-negative infections behind many dropsy cases a vet may choose a different class.
Not for human use. This product is for aquarium fish only. It isn’t approved for people. Do you want to know if fish antibiotics are safe for humans? Please read our guidance. Then talk to a doctor. Do not take this product.
How your order is reviewed and shipped
Add the quantity you need to the cart (1–12). Instead of a public checkout, 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 a vet-staffed operation keeps eyes on every order and confirms it’s intended for appropriate aquarium use. We email you at each step, reserved, confirmed, payment link sent, paid, and shipped. So you never have to sit and refresh a page.
GETTING THE MOST FROM AN ANTIBIOTIC
Antibiotics work on bacterial infections, not viruses — so the first step is simply confirming that’s what your pet has. That’s what the vet review here is for: matching the right antibiotic to the actual problem, at the right dose.
If a course is prescribed, giving all of it as directed — rather than stopping early once your pet looks better — is what fully clears the infection and keeps the medicine working well for next time. You clearly want to do right by your animal; this is just the way to get the best result from it.
Consistent with WHO and CDC guidance on responsible antibiotic use.
Frequently asked questions about cephalexin for fish
What does cephalexin (Fish Flex) treat in fish?
Cephalexin is a broad-spectrum cephalosporin used for external bacterial infections, fin and tail rot, skin ulcers and wounds, gill disease, and columnaris. A fish vet should confirm the cause and choose the drug and dose.
Is Fish Flex the same as Fish Keflex?
Yes, Keflex is a brand name for cephalexin, so "Fish Keflex," "Fish Flex," and "cephalexin for fish" all refer to the same molecule. Fish Flex Forte is simply the 500 mg strength.
Does cephalexin treat fin rot and columnaris?
These are common cephalexin indications, since it targets the external bacteria behind them. But similar signs can have other causes, so a vet should confirm before you treat, see our fin rot guide.
Is cephalexin 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.
Do I need a prescription?
This is an over-the-counter product for aquarium use. After you place an order, our pharmacy confirms your delivery details and sends a secure payment link before anything ships.
Is fish flex the same as Fish Flex Forte or Fix Flex?
Same active drug, different strength or brand label. Fish Flex is the 250 mg cephalexin capsule and Fish Flex Forte is the 500 mg version (Thomas Labs brand names); Fix Flex is a current cephalexin product at the same strengths. Dose from the label on the bottle you actually have.
Does fish flex come in 250 mg and 500 mg?
Yes, cephalexin for fish is sold as 250 mg (standard) and 500 mg (Forte) capsules of cephalexin monohydrate. Match the strength to your hospital-tank volume using the label-rate table above.
Where can I buy cephalexin for fish, do Walmart or Tractor Supply sell it?
General retailers don’t reliably carry aquarium cephalexin. A dedicated vet-staffed pharmacy is the more reliable route, with clear strength labels and fresh stock. You can reserve it here and our pharmacy confirms your order before it ships.
Related guides
Fish dropsy, often gram-negative, so cephalexin is usually NOT the first pick; a vet may choose a fluoroquinolone or aminoglycoside instead.
Fish popeye. The bulging-eye sign, its causes, and when antibiotics help.
Are fish antibiotics safe for humans?. A clear, no-sales answer.
How we reviewed this page
This product page is prepared under veterinary oversight following our editorial standards. The clinical statements, what cephalexin treats and where it’s 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.
