Acropora Care Guide: Light, Alkalinity, Pests and Your First Acro

You waited a year for your first acro. It goes in near the top of the rock, it looks great for a week, and by the third week it is either going brown or turning white from the base up. Acropora is not magic and it is not cursed. It punishes two things other corals forgive: water that moves around from day to day, and pests that ride in on the frag.

This guide is for the first acro: whether your tank is ready, how much light and flow it needs, the numbers that matter, the two pests every acro keeper meets, and what white or brown color is telling you. If you haven’t kept any small-polyp stony corals yet, start with our best beginner corals list instead.

What acropora is

Acropora is the most species-rich genus of stony coral, with about 150 described species. In the ocean it builds the tables, bottlebrushes, cushions and branching thickets you see in reef photos. Colonies grow as flat plates and tables, as dense cushions of short branches (the classic “millepora” look), as bottlebrushes with fine side branches, or as tall antler-like branches.

The quick way to tell an acro from a branching montipora: look at a branch tip. An acro has one larger polyp cup right at the tip, called the axial corallite, with smaller cups down the sides. A montipora doesn’t.

Florida has two native acros, staghorn (A. cervicornis) and elkhorn (A. palmata). Both are listed as threatened, and NOAA’s rules ban collecting, harming, selling, importing or exporting them. NOAA puts staghorn at less than 3 percent of its former abundance. Every acro a Florida store sells is an Indo-Pacific species, legally imported or grown from imported stock.

Acros from our listings.

Is your tank ready?

Sources disagree on how long a tank should run first. Chris Aslett says at least three months with a modest, stable fish load. World Wide Corals says six months. Tidal Gardens prefers a year. The number matters less than what the tank does week to week: a tank is ready when its alkalinity, calcium and magnesium hold steady between tests without big corrections, and when easier stony corals like montipora are already growing in it. Michael Paletta suggests montipora, pocillopora or stylophora as better first SPS for exactly that reason.

Stability: alkalinity is the one that bites

Tidal Gardens calls alkalinity “the parameter that fluctuates the most of the three and is the one that needs the most babysitting.” Paletta describes a daily alkalinity swing of 2 to 3 dKH as a significant stress. In his own 300-gallon system, some acros started bleaching from the base during a stretch of alkalinity swings. Once he tested more often and held alkalinity steady, the base bleaching stopped.

What that means in practice:

• Dose small amounts every day instead of large amounts every few days.
• Keep daily movement well under 1 dKH.
• When a number is off, bring it back over days, not hours. Paletta warns that fixing a problem fast can do as much harm as the problem.
• Make water changes small and regular. Aslett recommends no more than about 7 percent a week.

Our reef dosing guide covers the methods.

Small daily doses are easier with a dosing pump. Test often enough to see the daily movement.

Light: earn your way up

Tidal Gardens’ acros did best at about 300 PAR, and plenty of successful systems run over 500. Paletta puts 450 to 650 PAR as the range that brings out the most color in established SPS. A showcase acro tank featured in Reef Builders runs 750 PAR mid-tank.

A new frag should not start there. Tidal Gardens puts it bluntly: “More damage is caused by overexposure to light intensity than not providing enough light.” Start a new acro around 150 to 250 PAR, give it a couple of weeks, and move it up over the following weeks. Once it’s settled, leave it alone. Paletta says a moved coral needs at least two to three months to adjust.

Color follows blue light. Dana Riddle measured eight acro species and found green fluorescent protein rose sharply from 300 to 900 µmol, and that in A. millepora, blue light drove green and red fluorescence while red light did nothing noticeable. Paletta still recommends some white in the spectrum, since all-blue light washes out pinks and makes yellows look green.

Not every acro wants the top of the tank. A. russelli, a deep-water plating acro, wants 50 to 100 PAR and bleaches in a normal SPS tank. When you buy an acro, ask what light it was grown under.

Measure PAR at the frag before you move it up.

Flow

Strong, changing flow that reaches every branch, including the inside of the colony. Tidal Gardens lists lack of flow among the causes of tissue loss. As a colony fills in, it blocks flow to its own center and to its neighbors, so aim some flow into the colony as well as across it, and keep the pumps clean.

Water and nutrients: measurable, steady, never zero

Parameter	Target
Alkalinity	7 to 9 dKH in a low-nutrient tank, up to about 10 to 12 with higher nutrients. Held steady.
Calcium	400 to 450 ppm
Magnesium	1,250 to 1,350 ppm
Temperature	76 to 79°F, steady day and night
Nitrate	Measurable. Successful acro tanks run from about 0.5 to 25 ppm.
Phosphate	Measurable, roughly 0.02 to 0.2 ppm

Most care sheets still quote Randy Holmes-Farley’s 2004 targets: nitrate under 0.2 ppm and phosphate under 0.03 ppm. He has since updated his article. His current recommendation is 5 to 50 ppm nitrate and 0.06 to 0.3 ppm phosphate, and he warns that driving phosphate too low can give you pale corals. Paletta makes the same point: nobody he knows recommends zero nitrate and phosphate, because color and health both suffer. A 2013 study in Nature Climate Change found nitrogen without enough phosphate makes corals more prone to bleaching.

Pair the numbers. Low nutrients go with lower alkalinity. Holmes-Farley notes that SPS in ultra-low-nutrient tanks get burned tips when alkalinity is too high or moves too much. See our reef tank water parameters guide.

Feeding

Acros live mostly on light. A steady fish load helps; Tidal Gardens finds that more fish in the tank has a positive effect on acropora. Small foods like rotifers, cyclops and amino acids are optional. Overfeeding causes more trouble than underfeeding.

Amino acids, rotifers and cyclops: the optional foods this section mentions.

The two pests every acro keeper meets

Acropora-eating flatworms (AEFW)

They look like small brown ovals or bits of clear plastic lying flat against the coral, and they are hard to see on a stressed coral that has already browned. The signs are an acro going tan-brown with small white bite marks. They lay clusters of eggs on bare skeleton and at the base of the frag plug.

The eggs are the problem. A 2019 study by Barton and colleagues measured how long they take to hatch: about 11 days at 81°F and about 15 days at 75°F. No dip kills the eggs reliably, so one dip only resets the clock. A second treatment has to land after the eggs hatch and before the new worms mature. The same research team found that a sixline wrasse ate every adult flatworm in a lab test and none of the eggs. A wrasse helps. It isn’t a cure.

Red bugs

Tiny copepods, Tegastes acroporanus, that live only on acros. They are easiest to spot as yellow bodies with a red dot. An infested acro keeps its polyps pulled in. Tidal Gardens finds ordinary coral dips generally don’t work on them. The treatments that do are prescription veterinary drugs that also kill shrimp, crabs and pods, so talk to us or another store before you try one.

Crabs

Cultured acros often carry small crabs. Smooth ones with one big claw are usually guard crabs and can stay. Pull out any crab with a knobby shell, hairy legs and blue eyes; those eat coral tissue.

One dip resets the clock. Plan a second after the eggs hatch.

Why is my acropora turning white or brown?

Brown

There are four common causes, and they call for different fixes:

1. Too little light. Extra algae in the tissue show through.
2. A sudden jump to very high light. Paletta’s acros browned after a couple of weeks at 1,250-plus PAR, then bleached.
3. Extra nutrients.
4. Flatworms. Check for AEFW before you touch the lights.

White at the base

Tissue dying from the bottom up, leaving white skeleton below a clean line, is the pattern Paletta tied to alkalinity swings in his own tank. It can also be STN, which he describes as tissue loss that creeps up the colony over days or weeks. Check your alkalinity history first, then look for pests.

White or pale tips

Pale growth tips on an otherwise healthy branch are common; NOAA describes healthy wild staghorn as golden tan with white tips. Worry when white spreads down the branch, or when pale tips show up in a tank running very low nutrients and high alkalinity. The fix there is to lower alkalinity slowly and let nitrate and phosphate come up to measurable levels.

Tissue peeling off fast (RTN)

Cut healthy branches off well above the dying line and glue them to new plugs. Then check alkalinity and temperature history. Jake Adams adds a useful note: a small bare patch on one coral while everything else thrives usually isn’t RTN, so don’t change everything in the tank over it.

Fragging and growth

Once settled, acros are among the fastest-growing stony corals. In the wild, NOAA puts staghorn at up to 8 inches of branch growth a year. In a tank, expect weeks of little visible change while a new frag adjusts, then steady growth. We couldn’t find a reliable inches-per-month figure for home tanks.

To frag, use bone cutters in the tank or a band saw for a big colony, dry the base and the plug, and glue. Size matters: Paletta puts survival at about 25 percent for a 1/2-inch frag, about 50 percent at 3/4 inch, and 75 percent or better at 1 inch. Those are his estimates from experience, not a study. Cut bigger than you think you need.

Acros are aggressive toward nearby stony corals, and a growing colony shades and blocks flow to its neighbors. Give each one room.

Aquacultured acro frags from our tanks, and glue for mounting.

Quick answers

Are acropora hard to keep?

Harder than most. Tidal Gardens and World Wide Corals both advise against them for beginners. With a stable, mature tank and a pest routine for every new coral, they are very keepable.

Is acropora SPS or LPS?

SPS. It is a small-polyp stony coral in the family Acroporidae, the same family as montipora.

Acropora vs. montipora?

Same family. Montipora is more forgiving, grows as plates, crusts or branches, and has no axial corallite at the tips. It is the usual step before acros.

Do acropora need white light?

Blue light drives most of their fluorescent color, but every successful schedule we found includes some white.

Do acropora sting each other?

Yes. Tidal Gardens calls them an aggressive coral with strong stings toward nearby SPS. Space them out.

Sources

• Tidal Gardens, Acropora coral care and coral pests
• Michael Paletta, Reef Builders: calcium, alkalinity and magnesium, my corals are dying, why?, growing colorful SPS, managing tissue necrosis
• Randy Holmes-Farley, optimal parameters for a coral reef aquarium (updated 2022)
• Barton et al. (2019), AEFW life cycle and temperature, Frontiers in Marine Science, and Barton et al. (2020), predators of AEFW, Aquaculture Environment Interactions
• Dana Riddle, Reefs.com, making corals colorful: Acropora
• Chris Aslett, Reefs.com, SPS Academy, part I
• NOAA Fisheries, staghorn coral and elkhorn coral
• Wiedenmann et al. (2013), nutrient imbalance and coral bleaching, Nature Climate Change

The Underwater Gardener grows aquacultured corals in Key West, Florida (FL Aquaculture Certificate #AQ0254072). Our corals are raised under stable reef lighting in our own systems, so they arrive already adapted to a well-run tank. Corals ship UPS Next Day Air every Tuesday and Wednesday.