# Fish Antibiotics Compared: Which Class Treats What

> Choose an aquarium antibiotic by the type of bacteria. Penicillins and cephalosporins work best on gram-positive bacteria, fluoroquinolones on gram-negative bacteria, and tetracyclines and sulfa combinations have a broad range. Antibiotics do not treat parasites or fungi, and metronidazole is used mainly for internal protozoa.

Source: https://petremedyusa.com/guides/fish-antibiotics-compared/ · Updated 2026-09-30 · PetRemedyUSA

## Gram-positive vs gram-negative in one minute

Bacteria are grouped by cell wall structure. Gram-positive bacteria have a thick outer wall, while gram-negative bacteria have a thinner wall protected by an outer membrane. Many serious aquarium diseases, including columnaris, *Aeromonas* and *Pseudomonas* infections, are gram-negative. That is why the class you choose matters.

## Quick comparison table

| Class | Examples | Strongest against | Common aquarium uses |
| --- | --- | --- | --- |
| Penicillins | [Amoxicillin](https://petremedyusa.com/medications/amoxicillin-for-fish/), [Penicillin](https://petremedyusa.com/medications/penicillin-for-fish/) | Gram-positive | Fin rot, wounds, skin infections |
| Penicillin + inhibitor | [Amoxicillin/clavulanate](https://petremedyusa.com/medications/amoxicillin-clavulanate-for-fish/) | Resistant gram-positive and gram-negative | Infections not helped by plain amoxicillin |
| Cephalosporins | [Cephalexin](https://petremedyusa.com/medications/cephalexin-for-fish/) | Gram-positive, some gram-negative | Skin, fin and wound infections |
| Fluoroquinolones | [Ciprofloxacin](https://petremedyusa.com/medications/ciprofloxacin-for-fish/), [Levofloxacin](https://petremedyusa.com/medications/levofloxacin-for-fish/) | Gram-negative | Aggressive infections, columnaris-type disease |
| Tetracyclines | [Doxycycline](https://petremedyusa.com/medications/doxycycline-for-fish/), [Minocycline](https://petremedyusa.com/medications/minocycline-for-fish/) | Broad-spectrum | Internal and systemic infections, dropsy |
| Sulfonamides | [Sulfamethoxazole/trimethoprim](https://petremedyusa.com/medications/sulfa-antibiotics-for-fish/) | Broad-spectrum | General infections, second-line option |
| Lincosamides | [Clindamycin](https://petremedyusa.com/medications/clindamycin-for-fish/) | Anaerobes, gram-positive | Specialized cases |
| Macrolides | [Erythromycin](https://petremedyusa.com/medications/erythromycin-for-fish/) | Gram-positive | Mild external infections |

## What antibiotics will not treat

- **External parasites** such as ich (white spot) and velvet need parasite treatments, not antibiotics. See our [ich and velvet guide](https://petremedyusa.com/guides/ich-and-velvet-guide/).
- **Internal protozoa** such as Hexamita are treated with [metronidazole](https://petremedyusa.com/medications/metronidazole-for-fish/), which is an antiprotozoal.
- **Fungus** requires an [antifungal](https://petremedyusa.com/shop/antifungals/). Note that cotton-like growth is often bacterial columnaris, covered in our [columnaris vs fungus guide](https://petremedyusa.com/guides/columnaris-vs-fungus/).

## How keepers usually decide

1. Identify the symptoms and, if possible, whether the infection is external or internal.
2. Match the likely bacteria type (gram-positive vs gram-negative) to a class.
3. Treat in a [hospital tank](https://petremedyusa.com/guides/hospital-tank-setup/) for the full labeled duration.
4. If there is no improvement after a reasonable period, consult a veterinarian and consider a different class rather than repeating the same drug.

## Using treatment packs

Because the cause is often uncertain, many keepers keep a first-line drug and a backup from a different class on hand. Our [treatment packs](https://petremedyusa.com/shop/treatment-packs/) bundle complementary medications so a second class is already in the cabinet.

> **Responsible use.** Use antibiotics only for bacterial infections, follow label directions, and never use aquarium medications on humans or on fish intended for food. When in doubt, ask an aquatic veterinarian.

## Frequently asked questions

### What is the difference between gram-positive and gram-negative bacteria?

They differ in cell wall structure. Gram-positive bacteria have a thick wall, and gram-negative bacteria have a thin wall inside an outer membrane. Some antibiotics work better on one group than the other.

### Can I mix two antibiotics?

Combining medications can stress fish and change how each drug works. Unless a veterinarian or the label directs it, treat with one class at a time and switch only if there is no improvement.

### Do antibiotics cure ich?

No. Ich is caused by a parasite, and antibiotics act only on bacteria. Ich is usually treated with a copper-based product or with heat and salt where the species tolerates it.

### Why should I use a hospital tank for antibiotics?

Antibiotics can damage the beneficial bacteria that keep a tank cycled. A separate hospital tank protects your main aquarium and makes medication easier to control.