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.
The full Q&A library answers the most-searched aquarium questions. Browse calculators, glossary, and disease database for related help.
Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.
Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.
Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.
Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.
Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.
How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.
What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.
How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.
Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.
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