Lateral Line Erosion
Symptoms
Pitted erosion along the lateral line — the sensory canal running down each side of the fish — and on the head. Lesions start as small pits and progressively widen, sometimes reaching down to bone. The wounds themselves don't typically kill but disfigure permanently if untreated. Common in tangs, angels, large cichlids (especially in captivity-stressed individuals).
Causes
A chronic syndrome, not a single pathogen. Strongly associated with: (1) Nutritional deficiency — lack of vitamins A, C, D, and calcium/phosphorus imbalance, especially in fish fed only dry pellets or seaweed. (2) Stray voltage in the tank from poorly-grounded equipment. (3) Chronic high nitrate. (4) Activated carbon use in some sensitive species (debated). Often follows a Hexamita infection that erodes initial pits which then fail to heal.
Treatment
Improve diet dramatically — frozen mysis, brine shrimp, gel foods with added vitamins (Selcon, VitaChem), high-quality seaweed (Nori) for herbivores, varied diet for omnivores. Drop nitrates below 10 ppm. Add a titanium grounding probe to eliminate stray voltage. Existing pits heal slowly over months once husbandry is fixed. If active erosion continues, rule out Hexamita with a metronidazole course.
Prevention
Vary the diet — never feed only one food. Vitamin-rich frozen foods 2-3x weekly even for primarily-pellet-fed fish. Test for stray voltage if multiple fish develop HLLE. Keep nitrates low.
Notes
HLLE is not contagious — only affected fish develop pits in a tank with shared water. Surface pits heal in months once nutrition is fixed; deep pits scar permanently. Most affected fish live full lives with cosmetic damage.