Marine Ich, Velvet, and Common Diseases

Cryptocaryon irritans (marine ich) and Amyloodinium ocellatum (velvet) are the two most common killers of marine fish. Both are parasites with a free-swimming infectious stage that can persist in your display tank indefinitely — even without fish, ich can survive as dormant cysts on substrate for weeks.

Once introduced, these parasites are nearly impossible to fully eradicate from an established reef tank without removing all fish and running the tank fallow for 76 days. A quarantine tank (QT) prevents this scenario entirely.

Ich presents as small white spots (1–2 mm) resembling salt grains on the fish's body and fins — especially visible on dark fish or transparent fins. Fish rub against rocks (flashing), breathe rapidly, and become lethargic.

The lifecycle: trophonts attach to fish and feed for 3–7 days → tomonts fall off and encyst on substrate → tomites (infectious stage) hatch and swim looking for a host. The free-swimming stage lasts 12–48 hours before it must find a host or die.

Treatment in QT: copper at therapeutic levels (0.35–0.38 ppm ionic copper, or per manufacturer if using Cupramine) for 30 days minimum, with copper levels checked twice daily using a proper colorimetric test.

Velvet is far more lethal than ich and progresses quickly. It appears as a fine gold or rust-coloured dust over the body (especially visible under side-lighting), causes rapid breathing, clamped fins, and fish often die within 5–7 days of visible symptoms.

Velvet can kill a tank full of fish before you realise what's happening. Any new fish that dies rapidly for unknown reasons should be assumed to have velvet.

Treatment: Chloroquine phosphate (30 mg/L for 30 days) is the most effective treatment. Copper also works but velvet is more resistant. Combine with total darkness — Amyloodinium requires light for photosynthesis (it's a dinoflagellate).

Brooklynella hostilis predominantly affects clownfish and moves very rapidly — a fish can go from healthy to dead in 24 hours. It appears as grey or white mucus sloughing off the body, heavy breathing, and loss of appetite.

Treatment: Formalin dips (37% formaldehyde solution, 25 ml/10L for 45–60 minutes with aeration) followed by treatment in QT with formalin + hyposalinity (1.012 for 30 days). Act immediately — there is no time to lose.

Head and lateral line erosion (HLLE) causes pitting and discolouration along the lateral line and face, primarily in tangs and angels. It's associated with activated carbon dust, nutritional deficiency (lack of vitamin C), and poor water quality. Switching to high-quality carbon, feeding enriched frozen food (Selcon, Vitachem), and improving water quality usually reverses it slowly.

Fin rot presents as ragged, disintegrating fins — start with pristine water quality; if bacterial, treat with Furan-2 or kanamycin in QT.

Pop-eye (exophthalmia) is usually bacterial; one swollen eye is often injury-related and resolves on its own. Both eyes swollen simultaneously suggests a systemic infection requiring antibiotics (Maracyn, Kanamycin) in QT.