Fin Rot
Symptoms
Fin edges fray, blacken, or develop a white milky margin. Tissue progressively erodes from the tip inward, sometimes reaching the body if untreated. Affected fins look ragged or shredded. In advanced cases the fin rays show as exposed white spikes. Lethargy and loss of appetite often accompany.
Causes
Opportunistic gram-negative bacteria (commonly Aeromonas hydrophila or Pseudomonas) that exist in every aquarium but only invade compromised tissue. Triggers: poor water quality (high ammonia or nitrite), fin damage from fin-nipping tankmates or sharp decor, prolonged stress, low temperatures for tropical species. The bacteria itself isn't the disease — the immune failure is.
Treatment
Step 1: large water change (25-50%) and find the husbandry problem (test ammonia, nitrite, nitrate). Most early cases reverse with clean water alone. Step 2 if it progresses: medicate with a broad-spectrum antibiotic for gram-negative bacteria (kanamycin, nitrofurazone, or erythromycin in combination depending on severity). Aquarium salt at 1 tsp per 5 gallons can help freshwater cases as a mild antibacterial. Hospital tank is gentler on the biofilter for full antibiotic courses.
Prevention
Keep nitrate under 20-40 ppm. Don't crowd. Remove fin-nippers from tanks with long-finned species (bettas, angelfish). Avoid sharp-edged decor. Quarantine new arrivals — they're the most common vector.
Notes
Don't confuse fin rot with normal fin-nipping wounds (jagged but healthy edges) or with the white tissue regrowth that follows healing (looks ragged for weeks but is reattaching, not necrotic).