1. Environmental Parameters Issues:
> - Check the water quality for factors like ammonia, nitrite, nitrate, pH, and temperature to make sure they are within suitable ranges for tropical fish.
> - Consider if any recent changes or stressors happened (overcrowding, new decorations, medications).
2. Oxygen Deficiency:
> - Insufficient oxygen dissolved in the water. Aeration devices can add more oxygen.
3. Infections or Injuries:
> - Bacterial infections can look like 'smudges' and may cause lethargy and bottom-dwelling. Treatment may be possible with appropriate medications.
> - Inspect for physical damage and isolate potentially aggressive community tank mates while they recover.
4. Parasitic Infection:
> - Parasitic infections of the skin or gills that might lead to discoloration, lethargy and fin issues.
Action
- Isolate the affected fish immediately to avoid potential transfer.
- If it seems that smudges originate from the decorations of substrate, move the fish to ensure it's not a simple color reflection.