Fish scratching against rocks, substrate, or decor is called "flashing" and almost always indicates an external irritant. Most likely: ich (Ichthyophthirius multifiliis). Look for tiny white salt-grain spots on fins + body. Treat with heat (raise temp to 86°F slowly over 24h, hold 14 days) + Ich-X or Aquarium Solutions Ich-X for freshwater. Saltwater ich (Cryptocaryon) does NOT respond to heat - requires copper or tank transfer. Velvet (Amyloodinium for SW, Oodinium for FW): gold or rust dust sheen, especially under light. Highly lethal in 48-72 hours. SW: copper at 2.0 ppm or chloroquine phosphate. FW: copper sulfate or Coppersafe. Flukes (Gyrodactylus, Dactylogyrus): no visible parasite but rapid breathing + flashing + occasional cloudy eyes. Praziquantel (PraziPro for FW or SW). Trichodina (FW): excess slime coat + flashing without spots. Formalin or ParaGuard. Non-disease causes: high ammonia or nitrite (test water first), pH crash, copper overdose. Always test water before assuming parasites.
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