Harlequin Rasboras with Red Fins After Water Change

90L community tank. After 20% water change, harlequin rasboras show red fins/tail, flared dorsals. Parameters: 0 ammonia/nitrite, pH 6.6–7.4. Asks if stress, territorial penguin tetras, or salt would help.

Diagnosis: Mild Bacterial Fin Erythema

Red fins = early bacterial infection (often Aeromonas/Pseudomonas), triggered by stress (water change, parameter shift). Not territorial aggression.

Action Plan

1. Test nitrate & GH/KH. High nitrate or hardness swings contribute.
2. Do NOT add salt. Rasboras are salt-intolerant.
3. Large water change (30–50%) with temp-matched, dechlorinated water.
4. Consider removing the pleco—high bioload elevates nitrates/organics.
5. If persists 48 hrs: Broad-spectrum antibiotic (e.g., Kanamycin) in quarantine.

Stress from water change + possible parameter mismatch (pH swing 6.6→7.4) triggered this. Stabilize parameters, reduce bioload, treat if needed.