# Bloat and Internal Parasites in Fish: Signs and Treatment

> Bloat and internal parasites often show as a swollen belly, white stringy waste, appetite loss and weight loss. Internal flagellates such as Hexamita are commonly treated with metronidazole, an antiprotozoal, while bacterial causes need an antibiotic. Improve diet and water quality, and treat in a hospital tank.

Source: https://petremedyusa.com/guides/bloat-and-internal-parasites/ · Updated 2026-09-30 · PetRemedyUSA

## Bloat vs dropsy

Bloat is a swollen belly that may come from constipation, diet, internal parasites or bacterial infection. Unlike [dropsy](https://petremedyusa.com/guides/dropsy-in-fish/), the scales usually lie flat. If scales stick out, treat it as dropsy.

## Signs of internal parasites

- White, stringy or clear waste
- Thin body with a hollow belly despite eating
- Loss of color and lethargy
- Pitting on the head or lateral line, common in cichlids and discus

## Common causes of bloat

- Overfeeding dry, expanding foods
- Constipation from a low-fiber diet
- Flagellate parasites such as *Hexamita* and *Spironucleus*
- Bacterial infection in the gut or body cavity

## What to try first

1. Fast the fish for a day or two, then offer blanched, deshelled peas or other fiber-rich food.
2. Keep water clean and stable.
3. If waste stays stringy or the fish loses weight, suspect parasites.

## Where metronidazole fits

[Metronidazole](https://petremedyusa.com/medications/metronidazole-for-fish/) is an antiprotozoal and antibacterial for anaerobes. It is widely used by aquarists for internal flagellates. Fish that refuse food can be treated through the water in a [hospital tank](https://petremedyusa.com/guides/hospital-tank-setup/), following the directions on the label.

## When bacteria are involved

If bloat comes with red patches, hemorrhaging or fast deterioration, a bacterial infection may be present and a systemic antibiotic such as [doxycycline](https://petremedyusa.com/medications/doxycycline-for-fish/) may be needed. Our [Internal Parasite and Bloat Pack](https://petremedyusa.com/shop/treatment-packs/) combines both approaches.

> **Reminder:** Metronidazole does not treat ich or velvet, which are external parasites.

## Frequently asked questions

### What causes bloat in fish?

Overfeeding, constipation, internal parasites and bacterial infections are the most common causes. Diet and water quality are the first things to review.

### How do I know if my fish has internal parasites?

White stringy waste, weight loss while still eating, pale color and, in some species, head pitting are typical signs.

### Can metronidazole be used for bloat?

It is commonly used when bloat is linked to internal protozoa or anaerobic bacteria. It will not help if the cause is constipation or a different pathogen.