Aquarium Acclimation Guide, Fish and Corals

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Proper acclimation is one of the most important things you can do for your new livestock. Your fish, corals and invertebrates have travelled in sealed bags, and the water inside them is not the same as the water in your aquarium. Temperature, pH and salinity will all differ. Introducing an animal too quickly forces it to adjust to those changes all at once, which causes stress and can be fatal. Taking your time here gives your new arrivals the best possible chance of settling in and thriving.

This guide covers freshwater fish, axolotls, marine fish, corals, clams and invertebrates, and settling in reptiles and frogs. Please read the section relevant to your purchase before you begin, and if you have bought more than one type of livestock, acclimate each group separately.

Before You Start

Test your aquarium water and make sure conditions are stable and suitable for the species you have purchased.
Have your equipment ready before you open any bags: a clean bucket or container used only for aquarium purposes, a soft non-abrasive net, and freshly prepared water at your aquarium's temperature to top the tank back up afterwards.
Turn off your aquarium lights and dim the room. Your livestock has spent the journey in darkness, and bright light on arrival is a major and unnecessary source of stress.
Work calmly and without rushing. Most acclimation should take around 45 to 60 minutes; corals and sensitive invertebrates benefit from longer.

An Important Note on Transport Water

The water your livestock arrives in should never be added to your aquarium. During transit, waste builds up in the bag and carbon dioxide keeps the water slightly acidic, which holds that waste in a less harmful form. The moment the bag is opened and fresh water is gradually introduced, the pH begins to rise and any accumulated ammonia rapidly becomes far more toxic. Acclimate in a separate container, keep the process moving at a steady pace, and always net your livestock out into the aquarium rather than pouring the transport water in. Discard the transport and acclimation water once you are finished.

Freshwater Fish

You will need: a soft non-abrasive net, a clean cup, a clean bucket or container, and freshly prepared freshwater at your aquarium's temperature.

Turn off the aquarium lights and dim the room.
Float the sealed, unopened bag in your aquarium for around 15 minutes so the temperature inside the bag matches your tank. Do not open the bag during this step.
Open the bag below the seal and gently pour the fish and transport water into your clean bucket, tilting it so the fish stays comfortably submerged.
Add roughly one cup of aquarium water to the bucket every 5 minutes for around 45 minutes. This gradually adjusts the fish to your aquarium's pH and hardness.
Gently net the fish and release it into the aquarium. Do not add any of the bucket water to your tank.
Discard all acclimation water and top the aquarium back up with your freshly prepared freshwater.
Leave the lights off and the room dim for at least the rest of the day, and avoid feeding until the following day so the fish can settle.

Axolotls

Axolotls are fully aquatic amphibians and the cup acclimation method applies, but there is one critical difference from standard fish acclimation: temperature. Axolotls require cool water between 14 and 18°C and begin to experience serious stress above 20°C. Anything above 22°C is dangerous. This means the temperature-matching step is more important for axolotls than for almost any other animal we sell, and it means you need to be aware of the temperature of the room you are acclimating them in.

If your home is warm, the acclimation bucket will warm up during the process. Have a small amount of cool water or ice on hand to keep the bucket temperature stable, and work efficiently.

You will need: a clean cup, a clean bucket or container, freshly prepared dechlorinated water at 14 to 18°C, and a thermometer.

Turn off all lights and dim the room. Axolotls are sensitive to light and should never be exposed to bright conditions during acclimation.
Float the sealed, unopened bag in your tank for around 15 minutes to begin equalising temperature. Check the water temperature in your tank with a thermometer first to confirm it is in the correct range.
Open the bag and gently pour the axolotl and its transport water into a clean bucket.
Add roughly one cup of tank water to the bucket every 5 minutes for around 45 to 60 minutes. Monitor the bucket temperature throughout and ensure it stays below 20°C.
Gently net the axolotl and lower it into the tank. Never lift an axolotl by hand; support it fully in a net or wet your hands thoroughly before any contact.
Discard all acclimation water and top the tank up with freshly prepared, correctly tempered water.
Keep lighting off or very low for the rest of the day. Do not feed until the following day.

Always dechlorinate axolotl water. Chlorine and chloramine are harmful to their permeable skin, which absorbs substances directly from the water much like a frog's skin does.

Marine Fish

You will need: a soft non-abrasive net, a clean cup, a clean bucket or container, and freshly prepared saltwater matched to your aquarium's temperature and salinity.

Turn off the aquarium lights and dim the room.
Float the sealed, unopened bag in your aquarium for around 15 minutes to match the temperature. Do not open the bag during this step.
Open the bag below the seal and gently pour the fish and transport water into your clean bucket.
Add roughly one cup of aquarium water to the bucket every 5 minutes for around 45 minutes.
Net the fish and release it gently into the aquarium, leaving the transport and acclimation water behind.
Discard all acclimation water and top the aquarium back up with freshly prepared saltwater.
Keep the aquarium lights off and the room dim for at least 12 hours after introduction, and hold off feeding until the next day.

Wherever possible we strongly recommend quarantining new fish in a separate tank before adding them to an established aquarium. Quarantine dramatically reduces the risk of introducing disease and lets you observe feeding and condition first. For a full walkthrough, see our Fish Quarantine Guide.

Drip Acclimation (Recommended for Sensitive Species)

Drip acclimation introduces aquarium water very slowly and steadily, and is the gentlest method for marine fish, corals, clams and all invertebrates, as well as sensitive freshwater species such as shrimp, discus and wild-caught fish. You will need a length of airline tubing and, ideally, a drip or airline control valve.

Float the sealed bag for around 15 minutes to match temperature, then transfer your livestock and its transport water into a clean bucket positioned below the aquarium.
Start a siphon from the aquarium into the bucket using the airline tubing. Tie a loose knot in the tubing, or use a control valve, to slow the flow to roughly 1 to 2 drops per second.
Let the water drip steadily until the volume in the bucket has at least doubled. Allow around 45 minutes for fish, and 60 to 90 minutes for corals and invertebrates.
Once acclimated, net or carefully transfer your livestock into the aquarium and discard all remaining water.
Top the aquarium back up with freshly prepared water and keep lighting low.

Echinoderms such as starfish, sea urchins and sea cucumbers are especially sensitive to changes in salinity and should be given the slowest, longest acclimation of all. Never rush these animals.

Saltwater Invertebrates

Shrimp, snails, crabs, anemones, clams, starfish and urchins are more sensitive to shifts in salinity and pH than most fish, so we recommend the drip method above wherever possible. If you are using the cup method instead, extend it and slow it down.

You will need: a soft net, a clean cup, a clean bucket, and freshly prepared saltwater at matching temperature and salinity.

Turn off the aquarium lights and dim the room.
Float the sealed bag for around 15 minutes, then pour the invertebrate and its transport water into a clean bucket.
Add roughly one cup of aquarium water every 5 minutes for 45 to 60 minutes.
Carefully move the invertebrate to its chosen place in the aquarium. Snails, crabs and anemones should be lifted gently rather than netted; never expose an anemone, clam or similar animal to the air any more than necessary.
Discard all acclimation water and top the tank back up with freshly prepared saltwater.
Keep the lights off and the room dim for around 12 hours.

Corals and Clams

Corals and clams need the same gentle water acclimation as other livestock, plus two extra considerations: a dip to remove hitchhiking pests, and a period of light acclimation so they can adjust to your aquarium lighting.

Handling

Always handle corals and clams by their hard base, skeleton or frag plug. Avoid touching the soft tissue, polyps or a clam's mantle, and keep them out of the air as briefly as possible.

Water Acclimation

Float the sealed bag for around 15 minutes to match temperature.
Open the bag and gently place the coral or clam, with its water, into a clean container.
Add roughly one cup of aquarium water every 5 minutes for 45 to 60 minutes, or drip acclimate as described above.

Coral Dip (Recommended)

Dipping is strongly recommended for corals to remove pests such as flatworms, nudibranchs and unwanted eggs before they can enter your display. Clams should only be dipped where the product specifically states it is safe to do so.

Prepare a dip container of aquarium water and add a coral dip product at the manufacturer's recommended dose. A second container of clean aquarium water is used to rinse afterwards.
Wear gloves and eye protection, as coral dips are strong solutions.
Place the coral into the dip for the time recommended on the product, gently swishing it to dislodge pests. A soft brush and reef-safe tools can be used to remove visible pests and to clean the frag plug.
Move the coral into the clean rinse container to remove any dip residue.
Place the coral or clam into the aquarium, and discard all dip and rinse water.

For a full breakdown of the dip products we stock and when to use each one, see our Coral Dipping Guide.

Light Acclimation

Your coral has spent its journey in darkness and will not tolerate going straight under full-intensity aquarium lighting, which can bleach or burn it within a day or two. Place the coral in a lower-light, lower-flow position toward the bottom of the tank for the first several days, then move it up in small steps over one to two weeks until it reaches its intended position. If your lighting is controllable, reduce intensity by around half and raise it back to normal in small increments over several weeks. As a general guide: soft corals are the most forgiving, LPS corals prefer moderate light and flow, and SPS corals need the highest light and strongest flow. Every coral benefits from being brought up to its final conditions slowly regardless of type.

Reptiles and Frogs

Settling in reptiles and frogs is not water acclimation in the aquatic sense, but it is just as important. The single most common mistake is handling or disturbing a new animal before it has had time to establish itself in its enclosure. An animal that is stressed from transit and immediately handled, prodded or moved around is far more likely to go off food, develop respiratory issues, and take weeks longer to settle than one that is simply left alone.

Before the Animal Arrives

The enclosure must be fully set up and running at the correct temperatures, UVB output and humidity before the animal comes home. Not almost set up. Fully running and verified with thermometers. A reptile or frog arriving into an enclosure that is still warming up or that has a UVB lamp that has not been tested is starting its captive life under stress. Set up first, confirm everything is correct, then buy the animal.

Settling In

Place the animal directly into its enclosure as calmly and quickly as possible. Minimise handling time on arrival.
Leave it completely undisturbed for a minimum of five to seven days. This means no handling, no rearranging the enclosure, and no repeatedly opening the enclosure to check on it. Brief visual checks from a distance are fine.
Do not offer food for the first 48 to 72 hours. Most animals will not eat while settling in regardless, and offering food too early can increase stress rather than help. Once the animal is moving around normally and basking appropriately, begin offering food.
Keep the room quiet during the settling-in period where possible. Loud noise, vibration from speakers, and high foot traffic near the enclosure all delay settling.

Additional Notes for Frogs

Frogs absorb water, chemicals and substances directly through their skin. This is not just relevant to their water dish: it applies to handling as well. Never handle a frog with dry hands, and never handle with hands that have soap, sunscreen, insect repellent, lotion or any other product on them. Use clean, wet, powder-free hands or powder-free gloves. Always dechlorinate any water that goes into a frog enclosure, including misting water. Tap water chlorine and chloramine are absorbed directly through frog skin and are harmful even at low concentrations.

The five to seven day settling-in rule applies particularly firmly to frogs. Most will spend the first several days hidden and motionless. This is normal. The temptation to check on them constantly should be resisted.

For species-specific care information, see our Reptile Care Sheets and Frog Care Sheets.

Key Points to Remember

Always match temperature first by floating the sealed bag before you open it.
Never add transport or acclimation water to your aquarium. Always net or lift your livestock across.
Keep lighting low during acclimation and for several hours afterwards.
Axolotls need cool water (14 to 18°C) and must never exceed 20°C during acclimation.
Give corals and invertebrates more time than fish, and give echinoderms the most time of all.
Dip corals before adding them, and acclimate them to your lighting gradually.
Reptiles and frogs need five to seven days undisturbed before handling. Do not rush this.
Never handle frogs with unwashed hands or hands with any product on them.
Quarantine new fish where you can, and avoid feeding until the day after introduction.
Patience and stability are the two things that most improve survival and long-term health.

If you are ever unsure about acclimating a particular animal, please get in touch before you begin. We are always happy to help. For details on our arrive-alive coverage, please see our DOA / DAA Policy, and to see how your order is prepared for its journey, read How We Pack Your Fish, Corals and Plants.