How do I acclimate new fish to my aquarium?

Short answer

Drip acclimate over 60-90 minutes for marine fish, 30-45 minutes for freshwater. Float the bag for temperature first (15-20 min), then start a slow drip into the bag (2-3 drops/sec) until the bag volume has tripled. Net the fish into the tank - never pour the shipping water in.

In depth

Acclimation matters more than almost any other factor in whether a newly arrived fish survives the first 72 hours. Get acclimation wrong and you trigger osmotic shock, ammonia spikes, or pH crashes that kill specimens that arrived in perfect health.

Drip acclimation (the gold standard)

1. Open the box in a dim room. Lights stress fish further during acclimation.
2. Float the unopened bag in the tank for 15-20 minutes to equalize temperature only. Don't skip this.
3. Empty the bag (water + fish) into a clean acclimation bucket or container that fits the volume. NEVER use a container that has had soap, food, or chemicals in it.
4. Set up a drip line from the display tank to the bucket using airline tubing with a control valve - target 2-3 drops per second.
5. Drip until the bucket water volume has doubled or tripled. For marine: 60-90 minutes. For freshwater: 30-45 minutes.
6. Net the fish from the bucket into the tank. Do not pour shipping water into the tank - it carries ammonia, parasites, and pathogens you don't want in your display.

What changes by species

Sensitive marine fish (anthias, mandarins, dragonets, harlequin tuskfish): drip 90+ minutes. Some keepers triple-drip (water out, water in, repeat) over 2 hours.

Coral and sensitive inverts (cleaner shrimp, seahorses): drip 60-90 minutes minimum. Shrimp are particularly sensitive to specific gravity swings - if the bag SG is off from your display by more than 0.001, drip 2 hours.

Hardy freshwater fish (most tetras, danios, livebearers): 30 minutes is sufficient. Some keepers skip the drip entirely and just float-and-net for very hardy species.

Plants and dry goods: no acclimation needed. Plant immediately or let plants float for 24 hours before planting if they came tissue-cultured.

More questions

Can I just float the bag and pour the fish in?

You can, but you're trading 90% survival for 60-70%. Float-and-pour works only for very hardy species (livebearers, hardy tetras) and risks osmotic shock + ammonia exposure for sensitive species.

How do I drip acclimate without an airline?

Use a measuring cup to manually transfer 1/4 cup from the display every 5 minutes. Same end result, more babysitting. Most acclimation kits sold by aquarium retailers include the airline + valve - it's a $10 investment that pays for itself on the first sensitive specimen.