Excess Slime Coat and Grey, Cloudy Skin on Fish: Causes

Why fish turn grey or slimy: water irritants such as low pH, ammonia or chlorine, and ectoparasites such as Ichthyobodo and Chilodonella, with the tests and microscope checks to do before treating.

A milky, grey or blue-grey sheen on a fish is almost always extra mucus, produced because the skin or gills are being irritated. The irritant is either in the water (a falling or extreme pH, ammonia, chlorine, cleaning residues) or living on the fish (most often the protozoa Ichthyobodo and Chilodonella, or skin flukes). Excess mucus is a nonspecific sign, so the cause has to be found by water testing and a microscope, not by appearance alone.

What the symptom looks like

Practical Fishkeeping describes slimy skin as a milky or slightly grey sheen, in patches or over the whole body; in severe cases the slime sloughs off in sheets or strands. Fish often flash (rub against objects), lose appetite, become listless and, when the gills are involved, breathe fast or gasp at the surface. Mucus on the gills itself hinders breathing. Grey-white fluffy tufts are a different sign, pointing to fungal growth, and grey ulcers to bacterial infection.

Water irritants: test these first

Mucus is the skin's first defence against irritation, and several common water problems trigger it:

• Low or falling pH: a gradual pH decline makes fish secrete excess mucus, seen as a slimy skin, and mucus on the gills causes rapid gill beats and gasping. Very low pH can also cause tremors, darting and jumping. The ph-keeps-dropping guide covers why pH falls and how to stabilise it.
• Ammonia: Practical Fishkeeping lists over-production of mucus, seen as opaque patches on the skin, among the signs of ammonia poisoning.
• Chlorine and chloramine: the MSD Veterinary Manual lists excess mucus on skin and gills, cloudy corneas, ragged fins and lethargy as signs of chronic exposure.
• Detergents and other chemical irritants: in a five-month study, a biodegradable detergent increased mucus release and mucus deposits on the gill rakers of Astyanax altiparanae.

Test total ammonia, nitrite, pH and carbonate hardness (KH), and chlorine if a water change was recent. Ammonia and nitrite should read zero, and pH and KH should be stable from test to test. A low KH with a falling pH points to acidification; any ammonia or nitrite points to a filtration or overstocking problem.

Ichthyobodo (costia)

Ichthyobodo is a flagellated protozoan that lives on the skin and gills. It is so small (the Alaska Department of Fish and Game gives 5 to 10 micrometres) that it is easily missed even under the microscope. It is not host-specific: it parasitises a wide range of warm- and cold-water fish, mainly in fresh water, and fry and fingerlings are especially susceptible. In advanced cases a blue-grey film covers the fish, the result of increased mucus production and thickening of the epidermis, which the MSD manual calls blue slime disease. Infested fish are listless, refuse food, flash and may pipe at the surface; on the gills the parasite causes swelling, hyperplasia and lamellar fusion, and secondary bacterial and fungal infections are common.

Outbreaks are linked to crowding and poor sanitation. A field review of African fish published by FAO notes that the parasite feeds on epithelial cell contents, first causing excess mucus and then depleting the mucus cells, which leaves fish looking dry and open to further damage. The Ichthyobodo necator disease article covers treatment.

Chilodonella

Chilodonella species are heart-shaped ciliates that move slowly over the skin and gills. Heavily infested fish lose condition and show copious mucus where the parasites are most numerous; with gill involvement come rapid breathing, coughing and visibly swollen, mucoid gills, along with flashing and poor appetite. Two species are involved, C. piscicola and C. hexasticha. Cold stress is a known trigger: Practical Fishkeeping notes that the parasite is more frequent in ponds below 10 °C, and the FAO review reports massive infections at both 12 to 17 °C and 25 to 30 °C in overcrowded, stressed or poorly fed fish. As with Ichthyobodo, the mucus layer can be exhausted and the fish then look dry. Treatment is covered in the Chilodonella piscicola disease article.

Other parasites that cause slime

Skin flukes (Gyrodactylus) and the ciliate Trichodina produce the same picture of extra slime, scratching and agitated swimming. The MSD manual notes that Trichodina infections usually cause less mucus than Chilodonella and often indicate poor sanitation or overcrowding. Practical Fishkeeping points out that all of these organisms can live on fish at low numbers without causing signs and multiply once the immune system is compromised, so a water problem often comes first and the parasite outbreak follows.

Diagnostic steps before treating

1. Test ammonia, nitrite, pH, KH and, after a water change, chlorine. Correct any problem with partial water changes using dechlorinated water matched for temperature and pH.
2. Note recent events: new fish, a pH crash, cleaning with chemicals, a cold snap in a pond.
3. Have a skin scrape and a gill biopsy examined under a light microscope. The MSD manual stresses that fresh preparations with living parasites are often diagnostic; Ichthyobodo is recognised by its jerky, spiral swimming or a flickering motion when attached, Chilodonella by its slow spiral glide.
4. Use a live or freshly dead fish for examination, because the diagnosis relies on seeing living, moving parasites.
5. Treat only for the organism that is identified, for the full course.

If fish are also dying, the fish dying one by one troubleshooting guide and the new fish dying after purchase guide cover the wider causes, and the red or inflamed gills guide covers gill-specific signs.