Acclimating corals means slowly adjusting them from their shipping or store water to your tank’s water chemistry over 30 to 60 minutes, minimizing osmotic shock that can cause tissue damage or bleaching. The process involves temperature matching, gradual water mixing, a pest-prevention dip, and careful placement based on the coral’s light and flow needs.
Why Acclimation Matters
Corals sit in shipping bags or store water that can differ significantly from your tank in temperature, salinity, and pH. Rapid changes in any of these parameters force the coral’s cells to take on or lose water too quickly, stressing the tissue and potentially causing it to expel its symbiotic algae (the tiny organisms living inside coral tissue that provide it with food through photosynthesis). Once those algae are gone, the coral bleaches and will starve if it can’t recover.
There’s also a chemistry problem specific to shipping bags. As corals and their hitchhikers respire in a sealed bag, CO2 builds up and drives the pH down. That low pH actually keeps ammonia in a less toxic form. When you open the bag and expose the water to air, the CO2 off-gasses, pH rises, and ammonia rapidly converts to its toxic form, which becomes dangerous at concentrations as low as 0.2 ppm. This is why you don’t want to spend hours slowly drip-acclimating corals the way you might with fish: the longer the bag water sits open, the more toxic it becomes.
Step-by-Step Acclimation Process
If your corals were shipped, open the box in a dimly lit room. Corals that have been in darkness for 12 to 24 hours during transit can be shocked by sudden bright light. Keep your room lights low and your tank lights off or dimmed during the entire process.
Float the sealed bag in your tank or sump for about 15 minutes. This equalizes the temperature without introducing any bag water to your system. If you received multiple bags, float them all at the same time.
After the temperature matches, open the bag and pour the coral and its water into a clean container (a small bucket or plastic tub works well). Discard about half the bag water. Then add about half a cup of your tank water to the container every five minutes for 15 to 20 minutes. This gradually shifts the salinity and pH toward your tank parameters without a sudden jump. If the salinity difference between the bag water and your tank is large (say the bag reads 1.020 and your tank is 1.025 or 1.026), extend this process to 30 minutes with smaller additions.
Once you’ve finished mixing, remove the coral from the container. Never pour shipping or store water into your display tank, as it may carry parasites, bacteria, or excess nutrients.
Dipping for Pest Prevention
Before placing a new coral in your tank, a brief dip in a coral-safe solution removes hitchhikers like flatworms, nudibranchs, and pest snails that are nearly impossible to spot on a dry frag plug. This step isn’t optional if you want to protect your existing colonies.
Most general-purpose coral dips use plant extracts or potassium-based formulas. Mix the dip in a separate container using your tank water, place the coral in for 5 to 10 minutes (following the product’s instructions), and gently agitate it with a turkey baster or small powerhead to knock pests loose. You’ll often see tiny worms or other organisms fall off during the dip.
Iodine-based dips are effective against bacterial infections and parasites but require more caution. Recommended soak times vary by coral type: roughly 7 to 10 minutes for small-polyp stony corals, 6 to 9 minutes for large-polyp stony corals, 8 to 11 minutes for soft and leather corals, and 5 to 8 minutes for colonial polyps like zoanthids. Some corals are notably sensitive to iodine. Xenia, for example, can shrivel and stop pulsing with even moderate iodine exposure, and zoanthids, mushrooms, and ricordea have also shown vulnerability to overdosed iodine solutions. If you’re dipping a species you’re unsure about, stick with a gentler plant-extract dip or shorten the soak time.
For algae growing on frag plugs, a brief hydrogen peroxide dip (a 10:1 ratio of saltwater to 3% peroxide for about 5 minutes) can clean the plug without harming most corals. Some hobbyists dip just the plug itself, keeping the coral tissue above the peroxide line.
Placement by Coral Type
Where you place a coral in your tank is just as important as the water acclimation. Getting this wrong is one of the most common reasons new corals decline in the first few weeks.
Soft corals (leathers, mushrooms, Kenya trees) are the most forgiving. They tolerate lower light and moderate flow, so placing them on the lower half of your rockwork or in shaded areas gives them the best start. They can be moved higher later once they’ve settled in.
LPS corals (torch corals, hammers, brain corals) need moderate light and gentle to moderate flow. Too much direct flow can prevent their fleshy polyps from fully extending, and too much light too soon can bleach them. The middle zone of your tank is typically the sweet spot.
SPS corals (Acropora, Montipora, Stylophora) ultimately need high light and strong flow, but placing a new SPS frag at the top of your tank on day one is a recipe for bleaching. Start them in the middle or lower-middle of the tank and move them upward over the course of two to three weeks, raising them a few inches every several days. This gives the coral’s symbiotic algae time to adjust to your specific lighting spectrum and intensity.
Light Acclimation Schedule
Regardless of coral type, reduce your tank lights to about 50% intensity on the day you add new corals. Over the next 7 to 14 days, gradually increase the intensity back to your normal levels. If your light controller supports a ramp-up schedule, program incremental increases of 5 to 10% every two or three days. Corals that were shipped overnight may have been in total darkness for over 24 hours, and even corals from a local store were likely kept under different lighting than yours.
This matters because the symbiotic algae inside coral tissue can produce toxic levels of reactive oxygen when hit with more light than they’re adapted to. The coral responds by expelling those algae, which is the bleaching process. Gradual light increases give both the coral and its algae time to ramp up their protective mechanisms.
Signs of Successful Acclimation
A coral that’s acclimating well will begin extending its polyps within the first 24 to 72 hours. In LPS corals, you’ll see fleshy tentacles inflate and reach out. SPS corals will show tiny polyps fuzzing out from their branches, especially at night. Soft corals should open up and, in the case of pulsing species like xenia, resume their characteristic rhythmic movement.
Signs of stress include tissue recession (the flesh pulling back to expose white skeleton), excessive mucus production, failure to extend polyps after several days, or color fading. Mild stress responses like temporary retraction or slight color change during the first day or two are normal. Persistent problems beyond 48 to 72 hours usually mean something about the placement or water chemistry needs adjustment. Try moving the coral to a lower-light, lower-flow spot before assuming it’s a lost cause.
Quarantine Before Display
If you want maximum protection for an established tank, a separate quarantine tank lets you observe new corals for pests and disease before they go into your display. Professional coral farms like Tidal Gardens quarantine new arrivals for 60 to 70 days before moving them into production systems. For home hobbyists, even two to four weeks in a simple quarantine setup (a small tank with a heater, basic light, and a powerhead) gives you time to spot flatworms, montipora-eating nudibranchs, or acropora-eating flatworms that a single dip may not eliminate.
During quarantine, you can repeat dips weekly to catch pests at different life stages. The length of the quarantine matters less than what you do during it. Observing corals under white light with a magnifying glass, checking the underside of frag plugs, and performing multiple dips is far more protective than simply letting a coral sit in a separate tank untouched for two months.