Acclimating Fish

Bringing home new fish is always exciting—but improper acclimation can quickly turn that excitement into disaster. Many hobbyists are familiar with the old “float the bag” method, but while it helps match temperatures, it does little to prepare your fish for the often significant differences in water chemistry between the bag and your tank. A safer and more effective approach is slow acclimation through gradual mixing.

This guide walks you through the best practices for introducing new fish to your aquarium, while minimizing stress and avoiding common mistakes.

The traditional floating method only equalizes temperature. It doesn't address differences in pH, hardness, or salinity—any of which can shock a fish’s system if they change too quickly. Worse still, the longer fish remain in the bag, the more ammonia builds up. As oxygen is consumed and CO₂ rises, the pH can drop, keeping ammonia in a less toxic form. But once you open the bag and oxygen re-enters, pH rises and ammonia becomes more dangerous.

Instead of prolonging the fish’s time in poor conditions, it’s better to start the acclimation process right away.

Step-by-Step: Proper Fish Acclimation

Prepare a Clean Bucket or Container

As soon as you get home, open the fish bag and gently pour the contents—fish and all—into a clean bucket or small container. This will serve as your acclimation chamber.

Tip: Use a container that’s easy to clean and has never been exposed to soap or chemicals.

Add Tank Water Gradually

Now, begin slowly mixing your aquarium’s water into the container. There are two common methods:

• Drip Method: Start a siphon using airline tubing to create a slow drip from your aquarium to the bucket. Tie a knot in the tubing or use a valve to control the rate—about 2–4 drips per second is ideal. This method can take 1–2 hours but offers the gentlest transition.

• Cup Method: For most freshwater fish, this simple approach works well. Use a small cup to add a scoop of tank water to the bucket every 1–2 minutes. Continue for about 15–30 minutes, depending on how delicate the fish is and how different the water parameters are.

Your goal is to slowly dilute the bag water until the fish is swimming in mostly your tank water.

Transfer the Fish (But Not the Water)

Once acclimation is complete, gently net the fish and place it into your aquarium. Do not pour the acclimation water into your tank. The water from the store or shipping bag may carry pathogens, parasites, or unwanted chemicals. Dispose of the remaining water in the bucket safely.

Final Thoughts

Acclimating your fish properly might take a bit more time than floating the bag, but it dramatically reduces stress and the risk of shock. Remember: rapid changes in pH, temperature, or hardness are far more dangerous than slow, deliberate transitions.

By taking the time to do it right, you'll give your new fish the best chance at a healthy, happy start in their new home.