Skipping quarantine is the single most common mistake hobbyists make. A "healthy looking" fish carrying ich, velvet, or flukes will introduce the disease to your display tank within 7-21 days. Once a marine ich infection establishes in a display, you face a 76-day fishless period to clear it.
A 4-6 week quarantine in a separate tank with prophylactic treatment costs $75-250 to set up and prevents 95% of disease outbreaks. The math is overwhelming.
A 10-20 gallon tank, bare-bottom (no substrate - easier to medicate and clean), with a sponge filter cycled in advance, heater, and a few PVC pipe sections for hiding. Total cost: $75-150 for the equipment, plus $20-30 for cycled bio-media from your display.
Salinity (saltwater) or hardness (freshwater) and temperature should match your display tank within 0.001 SG or ±1°F. Sudden parameter shifts during quarantine compound the stress fish are already under from shipping.
New marine fish: drip 90+ minutes. New freshwater: 30-60 minutes. Net the fish into the QT - never pour shipping water in.
Marine: dose Cupramine or Copper Power to 0.40-0.50 ppm over 24-48 hours, maintain for 14-30 days. Test daily with a Hanna or Salifert copper kit. Freshwater: raise temperature to 86°F and treat with general cure (praziquantel + metronidazole) for 7 days.
Watch for spots (ich), gold dust (velvet), labored breathing (gill flukes), white stringy poop (internal parasites), refusal to eat, fin clamping. Any symptom triggers a second treatment cycle and extends quarantine.
For ich-only quarantine: move the fish to a fresh tank every 72 hours for 14 days (4 transfers total). The 72-hour cycle interrupts ich's life cycle without copper. Suitable for invertebrate-friendly quarantine but more labor-intensive.
After 4-6 weeks of zero symptoms and a final week off medication, do a full water change in the QT to flush copper, drip-acclimate the fish to display parameters, and net into the display tank.