Red, Inflamed or Swollen Gills in Fish: Causes and Tests

What gill colour and breathing rate reveal: chlorine, ammonia and nitrite injury, gill flukes and bacterial gill disease, the test that confirms each cause and the first safe steps.

Swollen or abnormally coloured gills almost always mean the gill surface is being irritated, either by something in the water (chlorine, ammonia, nitrite) or by organisms living on the gills (flukes, filamentous bacteria). Test the water first, because chemical causes are the most common, can be confirmed in minutes and must be corrected before any disease treatment can work.

What to look at: colour, swelling and breathing

Gills are only partly visible under the gill covers, so the clues are combined: the colour at the edge of the cover, whether the covers stay flared, and how fast they beat. Flared gill covers, rapid or exaggerated breathing, gasping at the surface (piping) and crowding near filter outlets or air stones all indicate that the gills are not exchanging oxygen well, whatever the cause.

• Cherry-red, swollen gills with laboured breathing: typical of chlorine toxicity.
• Chocolate-brown gills and dark blood, fish piping at the surface: nitrite (brown blood disease).
• Pale, swollen gills with excess mucus and fast breathing: gill flukes or bacterial gill disease, often on top of poor water quality.
• No visible change but lethargy, poor appetite or spinning: ammonia; gill thickening from ammonia is microscopic.
• Gasping at dawn, larger fish affected first: low dissolved oxygen.

Chlorine and chloramine

Municipal water is disinfected with chlorine or chloramine, and both are highly toxic to fish. According to the MSD Veterinary Manual, adverse effects begin at chlorine concentrations as low as 0.02 mg/L and deaths occur at 0.04 mg/L; the Zebrafish International Resource Center notes that tap water typically carries 0.5 to 1.0 mg/L. Cherry-red, swollen gills with acute necrosis of the gill tissue and rapid respiratory distress are characteristic, sometimes with pinpoint haemorrhages around the head. Chronic low-level exposure shows as excess mucus on skin and gills, ragged fins, cloudy eyes and lethargy.

Confirm with a chlorine test on the tank water and on untreated tap water. If the problem started right after a water change, the fish dying after a water change guide covers the other causes linked to new water.

Ammonia

Un-ionized ammonia (NH3) damages gill tissue at low concentrations. The MSD manual reports histological gill changes from 0.05 mg/L NH3, with thickening (hyperplasia and hypertrophy) of the gill epithelium and fusion of the lamellae, and lethal levels for many fish around 2 mg/L. A Southern Regional Aquaculture Center fact sheet similarly lists gill and kidney damage from about 0.06 mg/L. The toxic share of total ammonia rises with pH and temperature, so the same test reading is more dangerous in warm, alkaline water.

Ammonia damage itself is not visible without a microscope; outward signs are lethargy, poor appetite and, in acute cases, spinning, disorientation and convulsions. Test total ammonia together with pH and temperature. New tanks and tanks with a stalled cycle are the usual source; the cycle stall troubleshooting guide covers that situation.

Nitrite

Absorbed nitrite converts haemoglobin into methaemoglobin, which cannot carry oxygen. The diagnostic sign is a chocolate-brown colour of the gills and blood, and affected fish gather in oxygen-rich areas or pipe at the surface. The MSD manual lists toxicity from about 0.1 mg/L nitrite, and the Zebrafish International Resource Center advises keeping it below that level. Chloride in the water competes with nitrite at the gill and is protective; any deliberate addition of salt for this purpose should be set by a veterinarian for the species involved.

Gill flukes

Monogenean flatworms of the genus Dactylogyrus attach to the gills with two pairs of anchors and mainly infest freshwater cyprinids. The life cycle is direct, with no intermediate host, so numbers can build up quickly in a closed tank. Infections cause inflamed gills, heavy mucus secretion and accelerated breathing; fish may hold the gill covers open, rub against objects (flashing), swim at the surface, lose appetite and, in heavy infections, gasp. UF/IFAS notes that heavily infested gills become swollen and pale, and that crowding, poor nutrition and poor water quality favour outbreaks.

Flukes are identified only by microscopic examination of a gill or skin mucus biopsy, ideally from a live or freshly dead fish because the parasites do not survive long after death. Treatment is covered in the Dactylogyrus (gill flukes) disease article.

Bacterial gill disease

Bacterial gill disease is usually caused by filamentous bacteria of the genus Flavobacterium, most often F. branchiophilum, which are present at low levels in the water. Large numbers attached to the gills trigger proliferation of the gill epithelium, swollen lamellae and sometimes lamellar fusion or clubbing, which reduces gas exchange. The Alaska Department of Fish and Game describes pale, swollen gills, flared gill covers, exaggerated breathing, fish facing into the current, loss of appetite and extra mucus on the head and upper body. The disease is documented mainly in young fish kept at high density, and it is linked to overcrowding, excess ammonia, low oxygen and suspended organic matter. Diagnosis is microscopic; the bacterial gill disease article covers treatment.

Immediate safe actions

1. Test chlorine, total ammonia, nitrite, pH and temperature, and record the results.
2. If chlorine, ammonia or nitrite is detected, change water with dechlorinated water matched for temperature and pH, and repeat testing.
3. Increase aeration and surface movement; damaged gills take up oxygen poorly.
4. Reduce or pause feeding to lower the ammonia load until readings are back to zero.
5. If the water is clean but fish still breathe fast or have pale, swollen gills, arrange a gill examination for flukes and bacteria.

When several fish fail over days or weeks rather than at once, the fish dying one by one troubleshooting guide covers the slower causes.