Goldfish Tail Damage: Fin Rot or Aggression?

Broken, White-Edged Tail in New Tank

One week into a new professionally set up aquarium: one small fish dead, another with damaged tail (broken, white tips), not eating, hiding. Others eating less.

Immediate Diagnosis Steps

• Test water: Ammonia, nitrite, pH, hardness. New tanks often have ammonia spikes.
• Inspect filter intake: Strong suction can shred fins—add pre-filter sponge.
• Check for aggression: Chase marks, nipped fins on others.
• Look for fin rot: White/fuzzy edges, progressive fraying.

Treatment

1. 50% water change with dechlorinator.
2. Add aquarium salt: 1 tablespoon per 5 gallons.
3. Add MelaFix (tea tree oil) per label for tissue regeneration.
4. If fin rot confirmed (white fuzzy margin), add antibiotic (e.g., Kanaplex, Furan-2).
5. Remove carbon during treatment.

New Tank Syndrome

"Professionally set up" doesn't mean cycled. Without established biofilter, ammonia rises fast. Test daily, change water to keep ammonia <0.25 ppm, nitrite 0. Add bottled bacteria (Tetra SafeStart, Seachem Stability).

See Treatment Guide and Water Chemistry.