How to Acclimate New Fish: the Float and Drip Methods

Two things change inside that bag on the way home. The temperature creeps toward room temperature, and the water chemistry drifts as the fish breathes and the bag sits closed. By the time you get home, the bag water and your tank water can be different enough that a sudden switch stresses the fish hard. Temperature is the obvious one. The quieter one is the dissolved chemistry: pH and hardness, plus salinity in a marine tank. A fish moved too fast across a big chemistry gap can stop eating, clamp its fins, or simply not recover. Invertebrates and corals are even more sensitive, because they cannot tolerate a quick swing in salinity at all. The fix is the same idea every time. Give the fish a little while to meet the new water halfway before it has to live in it.

Before you open anything, float the sealed bag in your aquarium for fifteen to twenty minutes. This lets the bag water come up or down to your tank temperature without the fish having to do any work. Turn the tank lights off first, since a dark, quiet acclimation is far less stressful than a bright one. If you have a long drive home in an Erie winter, the bag can arrive genuinely cold, so do not skip this step. Floating handles temperature and only temperature. Once the bag feels close to tank temperature, you move on to the water chemistry, and that is where you pick a method.

For a hardy freshwater fish going into an established community tank, this is plenty. After the bag has floated and matched temperature, open it and roll the top down so it floats like a cup. Every five minutes, add a small scoop of your tank water into the bag. Repeat that for about twenty to thirty minutes, by which point the fish is breathing a blend that is mostly your tank water. Then comes the one rule everybody breaks: do not pour the bag into your tank. Net the fish out of the bag and release it into the aquarium, and pour the leftover bag water down the sink. That bag water can carry whatever was in the store system, and there is no reason to add it to your display.

Drip acclimation is the gold standard, and it is what we would use for saltwater fish, any invertebrate or coral, wild-caught species, and anything delicate or expensive. Pour the fish and its bag water into a clean bucket or container that sits lower than the tank. Run a length of airline tubing from the aquarium down into the bucket to start a slow siphon, and put a control valve on the line so you can dial the flow down to roughly two to four drips per second. Let it drip until the water in the bucket has about doubled or tripled, which usually takes forty-five minutes to an hour. That slow climb lets the fish adjust to your exact chemistry a drop at a time. When it is done, net the fish into the tank and discard the bucket water. A few dollars of airline tubing and a small valve is the whole rig, and it is the single best thing you can do for a sensitive new arrival.

If you keep a reef or any marine invertebrate, drip acclimation is not optional. Invertebrates like snails and shrimp react badly to a fast change in salinity, and corals are no different, so they need the slow drip even more than fish do. Go gentle and give them the full hour, or a bit longer for a touchy coral. It helps to know your tank salinity and the bag salinity going in, so a hydrometer earns its keep here. Everything else follows the same routine: drip until the volume has roughly tripled, then move the animal into the tank without letting that water in with it. Our saltwater starter guide covers the wider picture of getting a marine tank ready for its first livestock.

A handful of habits separate a smooth move-in from a dead fish. Keep the tank lights off through the whole process so the new arrival stays calm. Never pour bag or bucket water into your display, and always net the fish across instead. Do not feed a brand-new fish right away, since it is too stressed to eat and the uneaten food just fouls the water. Give it a few hours, or until the next day, before its first small meal. And if you can, acclimate a new fish into a quarantine tank rather than straight into your main display, so anything it brought home gets caught before it reaches your other fish. Our quarantine guide walks through how to set that up and why it is worth the trouble.