Choosing and using a fish antibacterial
This guide is general aquarium-keeping information. For a sick animal, the right call is always a consultation with an aquatic or avian veterinarian, who can identify the organism and advise on the correct product, dose and duration for your species and system.
1. Identify before you treat
Most "bacterial" symptoms in an aquarium — clamped fins, cloudy eyes, frayed fins, ulcers — track back to water quality first. Test ammonia, nitrite, nitrate and temperature before reaching for any medication. Correcting the tank often resolves the symptom on its own.
2. Use a hospital tank
Treat affected fish in a separate, filtered, heated quarantine tank. Antibacterials disrupt the nitrifying bacteria in a display tank's biofilter, which can trigger an ammonia spike that does more damage than the original problem.
3. Follow the label, finish the course
Each product's label carries its own directions for aquarium use. Stopping early because the fish "looks better" is how resistant bacteria are selected for. Complete the labelled course, then run fresh carbon and a water change to strip residual medication.
4. Antibacterial classes at a glance
| Product | Active ingredient | Class |
|---|---|---|
| Fish Mox | Amoxicillin | Penicillin (beta-lactam) |
| Fish Pen | Penicillin | Penicillin (beta-lactam) |
| Fish Flex | Cephalexin | Cephalosporin |
| Fish Doxy | Doxycycline | Tetracycline |
| Fish Zithro | Azithromycin | Macrolide |
| Fish Flox | Ciprofloxacin | Fluoroquinolone |
| Fish Zole | Metronidazole | Nitroimidazole |
| Fish Sulfa Forte | Sulfamethoxazole / Trimethoprim | Sulfonamide combination |
| Fish Cin | Clindamycin | Lincosamide |
5. Storage
Keep bottles sealed, dry and out of direct light at room temperature. Check the printed expiry before use and dispose of expired product through a pharmacy take-back programme rather than down the drain.
For aquarium and avian use only. Aqua Soma Labs products are sold for use in ornamental fish and pet birds. They are not for human consumption and are not intended to diagnose, treat, cure, or prevent any disease in humans. Keep out of reach of children and pets.
Where this guidance comes from
The general aquarium-keeping guidance on this page reflects published veterinary and regulatory sources rather than our own clinical opinion. Useful primary references:
- US FDA — Center for Veterinary Medicine, unapproved animal drugs: the regulatory status of over-the-counter antibacterials sold for ornamental fish.
- American Veterinary Medical Association — antimicrobial use and resistance: why finishing a course and avoiding unnecessary treatment matters.
- World Small Animal Veterinary Association: general standards for companion-animal care.
We are a retailer, not a veterinary practice. Nothing here is a diagnosis or a prescription, and we do not publish dosing instructions beyond what the manufacturer prints on the label.
Last reviewed: September 2026. We re-check this page each quarter and whenever a policy, price or product label changes.