Most DOAs don't happen in transit - they happen in the first 24 hours after you open the bag. Done right, acclimation is a 30-90 minute process that drops post-arrival mortality close to zero. Done wrong, you can osmotically shock or pH-shock a fish that arrived perfectly healthy.
Float the unopened sealed bag in the destination tank to equalize temperature. Do not open it yet. Skip this step if your tank is more than 5F different from the bag - in that case use a small specimen container with bag water and slow-drip acclimate over 60+ minutes.
Pour the bag contents (fish + water) into a small container at tank level or slightly below. Run a length of airline tubing from the tank to the container, tied in a loose knot to control drip rate. Target 2-4 drops per second. Continue until the container volume has at least doubled. For sensitive species (anthias, wrasses, anemones, shrimp), triple the volume.
Net the fish out of the acclimation container and into the tank. Do not pour the bag water into your tank. Bag water carries elevated ammonia and may carry parasites you don't want introduced. Discard the bag water.
Lights off for the next 4-6 hours minimum. Tank inhabitants are less aggressive in low light. Don't feed for 24 hours - the fish has elevated stress cortisol and digestion is impaired. Watch for clamped fins, rapid breathing, scratching against rocks - signs of stress or parasites.
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Browse aquacultured fish (lower DOA risk) →Float 15-20 min for temperature, then drip-acclimate 45-90 min for chemistry. Sensitive species (wrasses, anthias, shrimp, anemones) get the longer end of the range.
Ideally yes for marine. Set up a 10-20 gallon QT with sponge filter + heater. Acclimate the fish into QT, run prophylactic copper or formalin for 14-21 days, then move to the display. This prevents marine ich + velvet from wiping out a stocked tank.
Less critical than saltwater - freshwater chemistry differs less between sources - but still worth 30 minutes of float + drip. The temperature equalization is the most important part.
Most common cause: pH or salinity shock from too-fast acclimation. Second most common: ammonia poisoning from the tank not being fully cycled. Third: pre-existing disease that was masked by transport stress.
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