Coral dip: how to dip corals and which dip to use

Why dip corals

New corals bring passengers. Bulk Reef Supply's dipping guide lists red bugs, red flatworms, Acropora-eating flatworms, Montipora-eating nudibranchs, zoanthid-eating nudibranchs, zoanthid-eating spiders, sundial snails, bristleworms, nuisance algae, brown jelly and bacterial infections among the problems a coral dip is meant to catch. Most are small, camouflaged and easy to miss at the store:

• Acropora-eating flatworms (AEFW). Now named Prosthiostomum acroporae. They match the coral's colour, and Kate Rawlinson, who studies them, notes that their egg "capsules are glued onto the bare coral skeleton, normally near the base of the colony." See the Acropora care sheet for the bite marks to look for.
• Montipora-eating nudibranchs. Formally described in 2020 as Phestilla subodiosus, from Montipora fragments bought from aquarium stores and other hobbyists. Adults are 1.5 to 4 mm long, and the species "Preys on coral species in the genus Montipora." It didn't feed on Porites, Acropora or Echinophyllia. The authors note that "Aquarists report that damages caused by this species can cost hundreds of dollars (USD) per outbreak." More on the Montipora care sheet.
• Red bugs. Tegastes acroporanus, a parasitic copepod described from Acropora.
• Zoanthid pests. Aeolid nudibranchs that match zoanthid colours and hide at the base of the polyps, plus sea spiders and sundial snails, which CoralRx lists among its targets. See the zoanthid care sheet.
• Aiptasia. Not something a dip is sold to kill. Practical Fishkeeping's description of how they arrive: "They just creep in unnoticed on the rocks to which the corals are attached, or on live rock."

Dipping is cheap insurance. It's far easier to deal with a pest in a bowl than after it has spread through an established tank.

Coral dips and their label doses

These are the doses and times the makers publish, for one US gallon (3.8 L) of saltwater unless stated. Labels change, so check the bottle in your hand before you mix.

Two groups are worth separating. Pest dips (CoralRx, ReVive, Koral MD, Coral Clean) are sold to clean hitchhikers and parasites off the coral. Iodine dips are disinfectants: Seachem says Reef Dip "is effective against bacteria, fungus, and protozoans" and can be used before any sign of disease or on a diseased coral; it's the type of dip used on damaged tissue and brown jelly. Tropic Marin's Pro-Coral Cure is also iodine-based; Dana Riddle lists it at about 0.56% iodine.

Lugol's solution is the other iodine option people search for. Brightwell sells its Lugol's for advanced reefers "to dose iodine as well as to create a dip for corals and their allies," but the retail listings we read give no dip dilution, and the product is very strong (at least 126,803 ppm iodine, per its guaranteed analysis). Unless your bottle prints dip directions, use an iodine dip that does.

What dips kill, and what they don't

We found no published, independent head-to-head test of the commercial coral dips on real reef pests. The closest evidence:

• Riddle's bioassays. Riddle tested ReVive, MelaFix Marine and Pro-Coral Cure on freshwater pond animals standing in for coral pests. After a 15-minute dip, all three killed 94% or more of the unidentified crustaceans. The snails were stunned but recovered within 24 hours. That suggests dips are more reliable against small crustaceans such as red bugs than against snails, which are better picked off by hand.
• The Barton trial on AEFW. A 2020 controlled study immersed infested Acropora for an hour in one of two veterinary dewormers (neither is sold as a coral dip, so we don't give doses here), then gave each fragment a five-second shake. Immersion alone removed 75% and 90% of the flatworms; the shake raised removal to roughly 93–95%. Just handling and shaking corals in plain seawater removed 33%. Nothing reached 100%, and "The mortality of P. acroporae was not measured in this study." The lesson for any dip: agitation matters. The authors suggest combining immersion "with a small wavemaker."

Eggs are the gap. AEFW egg capsules sit on bare skeleton, and the team that mapped the flatworm's life cycle recommends treating the colony again "to target new hatchlings after they emerge from their protective egg capsules." In their experiments, eggs took about 26, 15, 11 and 9 days to hatch at 21, 24, 27 and 30 °C, and at 27 °C the young took about 35 days to reach sexual maturity. For a coral kept near 28 °C they suggest a second treatment 13 to 31 days after the first: after the eggs have hatched, but before the new worms can lay more. Scraping off egg capsules with a scalpel or razor may help, they add, though that wasn't tested.

The same logic applies to other pests. The Reefs.com profile of zoanthid-eating nudibranchs says to remove egg masses by hand and "repeat dips every 4 to 7 days until no pests are seen." And Bulk Reef Supply's simplest advice works for all of them: "Removing coral frags from the coral plug or mount they came on can really help reduce the risk of pests surviving the dip." As it puts it, "Plugs are very common carriers of eggs and pests."

How to dip corals, step by step

1. Get set up while the bag floats. Float the coral's bag in your tank to match temperature. Put on gloves and eye protection.
2. Fill two containers with tank water. One is the dip, one is the rinse. BRS uses a gallon in each, then tops the tank back up with new saltwater. Using tank water means temperature and salinity match what the coral is about to live in, and the labels ask for it: CoralRx says saltwater from the aquarium, Seachem says water removed from the source aquarium, and Brightwell says aged seawater, preferably old aquarium water, in a plastic container.
3. Measure the dip for that volume. Shake the bottle and add the label dose. One product per container: "NEVER mix coral dips in the same container and ALWAYS rinse corals before they go back into your display tank."
4. Add water movement. A small powerhead or air stone in the dip container. CoralRx's directions say to keep the water moving, and in the Barton trial the shake removed many flatworms the soak alone didn't.
5. Remove the coral's guests. Seachem says to remove commensal crabs and shrimp or shake them off before dipping, and Tropic Marin says to remove coral crabs. They can go back on the coral afterwards.
6. Take the frag off its plug if you can. Then the plug, and any eggs on it, never goes in the tank.
7. Dip for the label time. Baste the coral with a turkey baster to blast pests out of branches and from under polyps. Don't go past the label maximum.
8. Rinse. Move the coral to the rinse container and swish it, then baste it again in clean water.
9. Look at what came off. Check the bottom of the dip container for bugs, worms and nudibranchs. BRS: "Infected corals can be dipped regularly until no parasites are seen falling off in the dip water."
10. Throw the dip away. Don't reuse it, and never pour it into the display or sump. Clean both containers.
11. Plan the next dip. Put a date on the calendar for a second dip after any eggs will have hatched, or hold the coral in quarantine until then (see quarantine below).

Which corals tolerate which dips

The labels say which animals each dip is meant for:

• Koral MD and Koral MD Pro: "It is safe for dipping stony and soft corals, sea anemones, mushroom anemones and zoanthids."
• ReVive: made "for cleansing live stony and soft corals, zoanthids, and anemones."
• Seachem Reef Dip: "It is safe to use with both stony and soft corals", and for anemones. Not clams: "We do not recommend it for clams."
• Tropic Marin Coral Clean: hard and soft corals, according to a BRS staff answer.
• CoralRx Pro: its maker says only that it was "developed to be safe for your coral", without a list.

How a coral reacts matters as much as what the label says. Large-polyp stony corals pull in when dipped, which shelters pests: "Fleshy corals like LPS retract when dipped and may need multiple dips". Hammers, frogspawn and torches (see the hammer coral, frogspawn and torch coral care sheets) are typical. A stressed coral is a bigger risk than any species: BRS warns that "if the coral is in a weakened state, it may not handle a dip all that well regardless of the species." For a coral that's already bleached or receding, use the short end of the label time and watch it, or hold it in quarantine instead. Expect some reaction either way. Seachem says to watch for "adverse reactions such as excessive sliming", and Tropic Marin says "it can take up to two days for corals to open fully again after the bath." If the coral is pale rather than pest-ridden, the problem is probably not a hitchhiker; the coral bleaching guide covers the usual causes.

Zoanthids and Palythoa need extra care because some contain palytoxin. CDC investigators wrote that "Activities that could potentially produce aerosols (e.g., scrubbing or using hot water to remove zoanthids)" should be undertaken with caution. Dip zoanthids gently, with the gloves and eye protection on, and never clean them with hot water.

Safety

• Protect yourself. Brightwell: "Wear rubber gloves & goggles when handling!" Its label also says the dip is "Irritating to eyes and skin". BRS lists gloves and eye protection for any dip.
• Don't overdose. More dip is not a better dip. Koral MD Pro's label is blunt: "Professional Strength, do not overdose!" Seachem allows a double dose of Reef Dip only "In very severe cases".
• Don't reuse it. CoralRx: "Do not reuse the coral dip as parasite may release toxins." Brightwell and Two Little Fishies say their mixed dips should be discarded after an hour.
• Don't mix products. One dip per container, with a rinse between them.

Why we don't recommend insecticide dips

You will still find recipes for dipping corals in garden insecticide; BRS's guide mentions DIY dips including a "homebrew made from Bayer brand insecticides", without a recipe. We don't recommend them. Using a lawn product on coral is off-label: the EPA-accepted label for Bayer Advanced Complete Insect Killer for Soil and Turf (imidacloprid 0.72% and beta-cyfluthrin 0.36%) states, "This product is highly toxic to aquatic invertebrates and fish," and "Do not spray the product into fish pools, ponds, streams, or lakes." Corals are aquatic invertebrates, and the label has no directions for aquarium use. The purpose-made dips above have printed directions; use one of them.

Dip, then quarantine

A dip is one step. A separate tank is the one that catches what the dip missed. BRS: "A coral quarantine tank can greatly reduce the chances of introducing pests into your display tank." The AEFW researchers recommend keeping treated corals "in an isolated quarantine tank between treatments, if possible", and for a tank that has had flatworms, they estimate it needs to go without any live Acropora for about 21 days at 30 °C or 25 days at 24 °C: long enough for the last eggs to hatch and the hatchlings to starve.

If you are setting up for your first corals, the starting a reef tank checklist covers the quarantine tub and the order to add livestock in, and the AEFW glossary entry sums up the flatworm. Keep the quarantine tank's salinity and temperature matched to the display with the salinity and temperature references, so the move into the display isn't another stress.