Fish Acclimation
You’ve cycled the tank and the parameters look good — now the fish are home. Do not tip them straight in. A sudden jump in temperature or chemistry causes osmotic shock, which can damage gills and kill livestock. Acclimation is the slow hand-over from bag water to tank water.
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What you will need
Net the fish · Never pour the bag
Have these ready before you open the bag. Airline tubing is only needed for the drip method.
Gear
Water and testing
Why acclimation matters
Bag water ≠ tank water · Match both slowly
Shop bag water is almost never the same as your tank. You need to equalise two things before the fish goes in.
Fish are cold-blooded. A jump of even a few degrees can shock their system. Floating the sealed bag equalises heat before you mix water.
A fast change in minerals and pH can burn gills and organs. If store pH differs from yours by more than 0.5, use the drip method even for hardy fish.
Method 1: Float and fill
Hardy community fish · Danios, guppies, tetras
The standard method. Temperature first, then slowly mix tank water into the bag. Takes about 30–45 minutes.
Dim the lights
Turn the aquarium lights off. Bright light plus a new tank is extra stress.
Float the sealed bag
Place the still-sealed bag in the tank for 15–20 minutes. This equalises temperature only — do not open it yet.
Mix in tank water
Open the bag. Add about half a cup of tank water. Wait 5–10 minutes.
Repeat until mostly tank water
Add another half cup every 5–10 minutes until the bag is mostly your tank water. If the bag gets too full, scoop some water out and discard it — do not pour it into the tank.
Net the fish — do not pour
Gently net the fish into the tank. Never pour the bag water in. Shop water can contain ammonia, copper or medication.
Method 2: Drip acclimation
Sensitive livestock · Shrimp, discus, marine fish
Slower and gentler. Use this for delicate species, or whenever pH differs by more than 0.5. Takes about 45–90 minutes.
Empty into a clean bucket
Tip the fish and all the bag water into a clean, fish-safe bucket. Dim the lights in the room if you can.
Start a siphon
Run airline tubing from the tank into the bucket. Start a gentle siphon of tank water into the bucket.
Restrict to a slow drip
Tie a loose knot in the tubing, or use an airline valve, so it drips at about 2–4 drips per second. Not a stream.
Wait until the volume doubles
When the water in the bucket has doubled — or tripled for very sensitive livestock — the chemistry is close enough.
Net into the tank
Net the fish (or shrimp) into the aquarium. Discard the bucket water. Do not add it to the display tank.
After you release them
Lights off · No food on day one
The job is not finished when they hit the water. Give them a quiet first night.
Keep lights off
Leave the aquarium lights off for at least 4 hours, or overnight, so they can find cover and settle.
Observe
Watch for heavy breathing or frantic swimming. Hiding or sitting on the bottom for the first few hours is normal.
Hold the food
Do not feed on day one. They are usually too stressed to eat, and leftover food only fouls the water.
Critical tips
When to drip · Quarantine if you can
Check the pH
If the shop pH is more than 0.5 away from your tank, use the drip method for every species, not just the delicate ones.
Quarantine
Best practice is a separate quarantine tank for 2 weeks so you can spot disease before it reaches the display.
Need a hand?
If a fish looks distressed after release, bring a water sample in and we can test it with you. Slow is always safer than fast.
