The nine products we stock fall into seven pharmacological classes. Knowing what separates them will not tell you which one a particular fish needs — that requires identifying the organism, which is a veterinary job — but it does explain why these are different products rather than interchangeable ones, and why using the wrong class is worse than using nothing.
The classes, in plain terms
| Class | Products | What characterises it |
|---|---|---|
| Penicillins (beta-lactams) | Fish Mox (amoxicillin), Fish Pen (penicillin) | Interfere with construction of the bacterial cell wall. Long-established, and the class most affected by acquired resistance because of decades of use. |
| Cephalosporins | Fish Flex (cephalexin) | Also target the cell wall, with a different spectrum from the penicillins and more stability against some bacterial enzymes. |
| Tetracyclines | Fish Doxy (doxycycline) | Block bacterial protein synthesis. Binds strongly to calcium and magnesium, which is why water hardness affects it. |
| Macrolides | Fish Zithro (azithromycin) | Also a protein-synthesis inhibitor, with a different spectrum and a long tissue half-life. |
| Fluoroquinolones | Fish Flox (ciprofloxacin) | Interfere with bacterial DNA replication. Broad spectrum, and a class where stewardship matters particularly because of its importance in human medicine. |
| Sulfonamide combination | Fish Sulfa Forte (sulfamethoxazole/trimethoprim) | Two agents blocking consecutive steps in bacterial folate synthesis, which is why they are combined rather than used alone. |
| Nitroimidazoles | Fish Zole (metronidazole) | Active against anaerobic organisms and several protozoa, which makes it the odd one out in a list otherwise aimed at bacteria. |
| Lincosamides | Fish Cin (clindamycin) | Protein-synthesis inhibitor with an anaerobic emphasis. |
Why the class has to match the organism
Bacteria differ in cell-wall structure, in the enzymes they produce, and in whether they can survive without oxygen. A drug that disrupts cell-wall construction does nothing useful against an organism whose wall it cannot reach. An agent aimed at anaerobes is the wrong tool for an aerobic infection. Choosing by guesswork means a proportion of courses treat nothing at all — while still exposing the bacteria in your tank to a selective pressure that favours the resistant ones.
Resistance is the reason the rules exist
Every incomplete or unnecessary course makes the surviving population a little harder to treat next time. The AVMA's antimicrobial stewardship guidance comes down to a few principles that apply as much to an aquarium as to a farm: treat only when treatment is warranted, choose the narrowest effective agent, use it for the labelled duration, and do not substitute one class for another because it is the bottle you happen to have.
What we can and cannot tell you
We can tell you exactly what is in a bottle: one active ingredient, at a stated strength and count, sealed at the factory with its lot code and expiry printed on the label. We cannot diagnose your fish, and we do not publish dosing regimens beyond what the manufacturer prints. That is not evasion — it is the line between a supplier and a clinician, and getting it wrong would not help your animal.
"Broad spectrum" is not a synonym for "better"
A broad-spectrum agent affects a wider range of organisms, including the harmless ones. In an aquarium that means more collateral damage to the nitrifying bacteria in the filter and to the microbial community in the substrate, and a wider selective pressure for resistance. Where a diagnosis points to a narrow agent, the narrow agent is the better choice — for the fish and for everything else in the system.
Water chemistry changes how some classes behave
These products are not inert once they enter a tank. Tetracyclines bind to calcium and magnesium, so hard water reduces the fraction that stays available. Several actives degrade faster in bright light or at higher temperatures. Activated carbon strips medication from the water column indiscriminately. Organic load — uneaten food, waste, detritus — consumes and inactivates some agents. This is another reason a bare, clean, dimly lit hospital tank produces more predictable results than a planted display tank.
What "finish the course" means in practice
Bacterial populations are not uniform. The most susceptible organisms die first, which is why the visible improvement arrives well before the infection is cleared. Stopping at that point leaves the least susceptible survivors to repopulate, and the next course works less well. Completing the labelled duration, then removing residues with carbon and water changes, is how you avoid training the bacteria in your own system.
Storage, expiry and potency
Antibacterials degrade with heat, humidity and light. A bottle stored sealed, dry and at room temperature holds its labelled strength through to the printed expiry date; one that has been opened, decanted into an unlabelled container or left on a sunny windowsill is an unknown quantity, and an unknown dose is a bad dose. Keep the original bottle, keep the label with its lot code, and dispose of expired stock through a pharmacy take-back programme.
A decision framework that does not involve guessing
- Test the water. Rule out ammonia, nitrite, temperature and oxygen first.
- Isolate and observe. Move the affected animal to a quarantine tank and keep a written record for a few days unless it is deteriorating rapidly.
- Get a diagnosis. An aquatic or avian veterinarian can identify the organism, or tell you that the problem is not bacterial at all — parasites, fungus, injury and water quality account for a large share of what gets treated as infection.
- Use what the diagnosis indicates, at the labelled dose, for the labelled duration.
- Clean up afterwards. Carbon, water changes, and a period of observation in clean water before the fish returns to the display tank.
Why we will not shortcut this for you
We are asked regularly to recommend a product for a described symptom. We decline, and the reason is not caution for its own sake: symptom descriptions do not identify organisms, the wrong class treats nothing, and a course that treats nothing still contributes to resistance. What we can guarantee is what is in the bottle — one named active, at a stated strength and count, sealed and lot-coded — so that when a veterinarian tells you what your animal needs, you can be certain of what you are giving it.
Sources
- US FDA, Center for Veterinary Medicine — unapproved animal drugs
- AVMA — antimicrobial use and antimicrobial resistance
- US EPA — nutrient pollution (nitrogen cycle background)
For aquarium and avian use only. This page is general husbandry information, not veterinary advice, and not a dosing instruction. It is not for human use. For a sick animal, consult an aquatic or avian veterinarian who can identify the organism and advise on the correct product, dose and duration.
Last reviewed: September 2026. We re-check this page each quarter and whenever a product label or policy changes.