Aquarium fish antibiotics
Fish Doxy: Doxycycline for Fish (100 mg Capsules, 100 Count)
A broad-spectrum tetracycline for ornamental fish, 100 mg. 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.
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Quick answer: Fish doxy is doxycycline for fish. The same fish doxycycline sold under various names. It is a broad-spectrum tetracycline that treats some bacterial infections, including gram-negative ones a vet has identified. A fish vet should confirm the cause; drug choice and dose are the vet’s call, ideally guided by testing. 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.
Starting a course? Use our antibiotic treatment tracker to log each dose, stay on schedule, and know when it’s time to call the vet.
What fish doxy is
Fish doxy is the common name for doxycycline sold for aquarium fish. Doxycycline is a tetracycline, a broad-spectrum antibiotic that physicians and veterinarians both use. 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 doxycycline for the right reason. A vet confirms the cause and the dose. That care is the whole point.
| Active ingredient | Doxycycline (as hyclate) |
|---|---|
| Strength | 100 mg per capsule |
| Count | 100 capsules |
| Drug class | Tetracycline |
| 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 doxycycline, which is sensitive to light and 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 doxycycline can help
Doxycycline is broad-spectrum. It is active against many gram-negative and gram-positive bacteria, so a fish vet may choose it for infections that some narrower antibiotics miss.1 The pathogens behind common fish problems such as ulcers and hemorrhagic septicemia are often gram-negative Aeromonas, and doxycycline has published activity against them.2 When a vet confirms an infection it covers, it can be an effective choice. The specific drug and dose remain the vet’s decision, guided by testing.
Where it’s the wrong choice
Doxycycline doesn’t treat viral, fungal, or parasitic problems. Resistance also matters: some fish bacteria have grown resistant to tetracyclines, so it doesn’t work every time, and guessing wastes time your fish may not have.3 This is why a vet should confirm the cause and, where possible, test which antibiotic fits, rather than reaching for any broad antibiotic first.
Take care with water chemistry: doxycycline can bind to calcium and other minerals in hard water, which may lower how much reaches the fish. It can also affect your tank’s good filter bacteria. Check your water, and a fish vet should confirm the cause before you treat.
Fish doxy strengths and label rates by tank volume
Fish doxy is sold as doxycycline (doxycycline hyclate) in a 100 mg tablet. The same strength older keepers knew as Thomas Labs Fish Doxy, Aqua Doxy, or Bird Biotic, and sometimes shortened to “fish dox” or “bird doxy.” The quick-reference below scales the manufacturer’s label rate of roughly one 100 mg tablet per 15 gallons to common 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 (doxycycline) | 100 mg tablets |
|---|---|---|
| 15 gallons (57 L) | 100 mg | 1 |
| 30 gallons (114 L) | 200 mg | 2 |
| 45 gallons (170 L) | 300 mg | 3 |
| 60 gallons (227 L) | 400 mg | 4 |
| 75 gallons (284 L) | 500 mg | 5 |
Dose by the water volume of your hospital or quarantine tank, not by the number of fish, and pre-dissolve each tablet before adding it. The bottle label and your aquatic veterinarian always take priority over this quick reference. Fix Doxy is the current doxycycline product that replaced the discontinued Thomas Labs Fish Doxy, Aqua Doxy, and Bird Biotic lines at the same 100 mg strength.
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 fish doxy
What does fish doxy treat in fish?
Doxycycline is a broad-spectrum tetracycline. A vet may use it for some bacterial infections, including gram-negative ones a vet has identified. A fish vet should confirm the cause and choose the drug and dose, ideally guided by testing.
Is fish doxy 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.
Is fish doxy the same as human doxycycline?
The active ingredient is the same molecule, but this product is manufactured, labeled, and sold for ornamental aquarium fish only, and isn’t intended or approved for human use.
Do I need a prescription?
This is an over-the-counter product for aquarium use. After you add it to the cart, our pharmacy confirms your delivery details and sends a secure payment link before anything ships.
Does hard water affect it?
It can. Doxycycline binds to calcium and other minerals, so very hard water may reduce how much reaches the fish. Ask a fish vet about your water and the right approach.
Is Fix Doxy the same as Fish Doxy, Aqua Doxy, or Bird Biotic?
They are the same idea under current labeling. Fish Doxy (Thomas Labs), Aqua Doxy, and Bird Biotic were earlier doxycycline products for aquarium and aviary use; Fix Doxy is the current doxycycline hyclate 100 mg tablet that replaced them at the same strength. Follow the current bottle label.
Does fish doxy come in 500 mg?
No, fish doxycycline is sold as 100 mg tablets. A larger total amount is reached with multiple 100 mg tablets by tank volume (see the label-rate table above), never by exceeding the label schedule. Your aquatic vet confirms the right amount.
Where can I buy doxycycline for fish, do Walmart or Tractor Supply sell it?
General retailers like Walmart, Tractor Supply, and PetSmart don’t reliably carry aquarium doxycycline. 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. A gram-negative infection where a vet may reach for a drug like doxycycline, not amoxicillin.
Fin rot. A common gram-negative infection doxycycline is often chosen for, once a vet confirms the cause.
Columnaris. The fast-moving cotton-wool disease where a gram-negative-active antibiotic like doxycycline may fit.
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 doxycycline 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.
