How to Acclimate Fish Received in the Mail

Receiving live fish through the mail is exciting — but the moments after opening that box are critical. Fish endure significant stress during transit: fluctuating temperatures, oxygen depletion, ammonia build-up from waste, and the shock of handling. A proper acclimation process gives your new fish the best possible start and dramatically improves survival rates.

Step 1: Prepare Before the Box Arrives

Before your fish even arrive, make sure your tank is fully cycled, water parameters are stable, and the temperature is within the species' preferred range. Have your acclimation supplies ready: a clean bucket or container, an airline tube, an air stone (optional), a small net, and a bottle of water conditioner such as Seachem Prime.

Dim the lights in the room — bright light adds unnecessary stress to fish that have been in darkness during transit.

Step 2: Open the Box Carefully

Open the box in a calm, quiet area. Check the bags for any signs of damage or leaks. The water inside the bag may look cloudy or discoloured — this is normal and is usually due to ammonia from waste during transit.

Float the sealed bags in your aquarium for 15–20 minutes to equalise the water temperature inside the bag with your tank, preventing thermal shock.

⚠️ Important: The Ammonia Spike When You Open the Bag

Here’s something many fishkeepers don’t know: opening the bag triggers an ammonia spike, and it’s critical to act quickly.

During transit, fish waste produces ammonia. However, the sealed bag environment is high in CO₂, which keeps the water pH low and acidic. In acidic conditions, ammonia exists in its ionised form — ammonium (NH₄⁺) — which is relatively harmless to fish.

The moment you open the bag, CO₂ rapidly off-gasses and the pH rises. As pH increases, ammonium converts back into its un-ionised form — free ammonia (NH₃) — which is highly toxic to fish. This conversion can happen within minutes and can be lethal, especially to sensitive species.

To counteract this, add a dose of water conditioner (such as Seachem Prime) directly to the bag or bucket as soon as it is opened. Prime and similar products temporarily detoxify ammonia, buying your fish safe time during the acclimation process. Use the recommended dose based on the volume of water in the bag or bucket.

Step 3: Drip Acclimation (Recommended)

Drip acclimation is the gold standard for sensitive fish, shrimp, and invertebrates. It slowly introduces your tank water to the bag water, allowing fish to adjust to differences in pH, hardness, and other parameters.

1. After floating, open the bag and immediately add a dose of Prime (or equivalent water conditioner) to the water.
2. Pour the fish and bag water into a clean bucket or container.
3. Using an airline tube tied in a loose knot (to control flow), start a siphon from your tank into the bucket.
4. Aim for a drip rate of approximately 2–4 drops per second.
5. Allow the water volume in the bucket to double before moving on — this typically takes 30–60 minutes depending on the drip rate.
6. For very sensitive species (wild-caught fish, shrimp, or delicate species), allow the volume to triple.

Step 4: Transfer the Fish

Once acclimation is complete, use a net to gently transfer the fish into your aquarium. Do not pour the bag or bucket water into your tank — transit water can contain pathogens, parasites, or elevated ammonia that you don't want introduced to your established system.

For shrimp and very small fish, you can use a fine mesh net or carefully pour through a net over a separate container to catch them without injury.

Step 5: Post-Arrival Care

After adding your new fish, keep the lights low for the rest of the day. Avoid feeding for the first 24 hours — fish are unlikely to eat when stressed, and uneaten food will only foul the water. Monitor closely over the next 48–72 hours for signs of stress, disease, or aggression from existing tank mates.

It's also good practice to quarantine new arrivals in a separate tank for 2–4 weeks before introducing them to your main display. This protects your existing fish from potential pathogens the new fish may be carrying.

Common Mistakes to Avoid

• Skipping temperature equalisation — even a few degrees difference can cause thermal shock.
• Not adding a water conditioner when opening the bag — the pH shift as CO₂ off-gasses converts harmless ammonium into toxic ammonia. Always dose Prime immediately.
• Rushing the process — acclimation should never be hurried. Give your fish time.
• Adding bag water to your tank — always net the fish out; never pour the transit water in.
• Bright lights immediately — keep things dim and calm for the first few hours.
• Feeding too soon — wait at least 24 hours before offering food.

A Note on DOA (Dead on Arrival) Fish

Despite best efforts from breeders and couriers, occasionally a fish may not survive transit. If you receive a fish that has passed away, photograph the unopened bag immediately and contact us as soon as possible. We ask that you follow our DOA policy guidelines, which require notification within a 2Hrtimeframe of delivery.

With a little patience and the right technique, acclimating fish is straightforward. Take your time, trust the process, and your new arrivals will be settling in and thriving before you know it.