Dim the lights, keep transport time short and first equalise temperature. For locally purchased fish in fresh transport water, a short gradual mixing process can reduce abrupt chemistry changes. For long-shipped fish, prolonged open-bag acclimation can be risky because bag chemistry changes once opened; follow the supplier's method. Transfer fish with a net or container and avoid adding transport water to the display tank where practical.
Acclimation is not a single ritual. A fish carried home for 20 minutes from a local shop is in a different situation from a fish that has spent many hours in a sealed shipping bag. The safest method depends on transport time, the water difference and the species' sensitivity.
Before opening the bag
- Prepare the receiving tank first: stable temperature, cycled filter and suitable water parameters.
- Dim aquarium lights and reduce activity around the tank.
- Check the bag for obvious leaks, distressed fish or unusually warm/cold water.
- Do not leave fish sitting in a hot car or bright sun while you prepare equipment.
Temperature acclimation
For conventionally bagged fish, floating the sealed bag briefly can bring the transport water closer to aquarium temperature. RSPCA guidance describes floating the bag to equalise temperature before transfer.
Do not leave a sealed bag floating for an excessive period, and do not let the bag water spill into the tank.
Gradual water mixing for short local transport
Aquarium Co-Op describes a staged dilution method using repeated additions of aquarium water over roughly half an hour. The exact pace is less important than avoiding abrupt change and unnecessary delay.
- Move the fish and enough bag water to cover them into a clean specimen container or bucket.
- Add aquarium water gradually to increase the container volume.
- Pause between additions while watching the fish for distress.
- Repeat until a substantial portion of the water is from the receiving aquarium.
- Net or gently transfer the fish into the aquarium and discard the transport water.
Why long-shipped fish may need a different method
During long transport, carbon dioxide and waste accumulate in the sealed bag. Opening the bag changes gas exchange and pH, which can alter the toxicity of ammonia already present.
For shipped fish, many suppliers therefore recommend temperature matching followed by prompt transfer rather than a long open-bag drip process. Follow the shipper's instructions because they know how the fish were packed.
After the fish enters the tank
- Keep lights low for the remainder of the day where practical.
- Watch breathing, balance and hiding behaviour.
- Delay heavy feeding until the fish has settled.
- Test water if the aquarium is newly stocked or recently cycled.
- Quarantine new arrivals where practical before introducing them to an established population.
Related aquarium tools and guides
Frequently asked questions
How long should I float a fish bag?
For temperature equalisation, a short period is usually enough; RSPCA guidance mentions about half an hour. Avoid treating a fixed time as mandatory if the bag is overheating or the fish is distressed.
Should I pour pet-shop water into my aquarium?
It is generally better to transfer the fish and discard transport water where practical, reducing the chance of importing waste, medications or pathogens.
Is drip acclimation always best?
No. It can be useful for some sensitive animals and water-chemistry differences, but prolonged acclimation can be inappropriate for fish that have spent many hours in shipping water.
Sources and further reading
This guide separates general aquarium arithmetic from fish-welfare decisions. For species-specific limits, always check the needs of the fish you intend to keep.