Best Placement for Acropora: PAR, Flow and Space Requirements

Acropora is the least forgiving genus in the reef tank, and most losses trace back to one thing: placing a colony or frag in a spot that doesn't match its light and flow needs. This guide gives you a formula — light zone, flow zone, and spacing — you can apply to your own tank shape and lighting setup, rather than a one-size-fits-all shelf recommendation.

PAR Basics: What You're Actually Measuring

PAR (Photosynthetically Active Radiation) measures the intensity of light in the wavelength range corals' symbiotic algae actually use for photosynthesis. It's measured in micromoles of photons per square meter per second, using a PAR meter (quantum sensor) held at the coral's surface.

PAR readings are a guideline, not an exact science — two meters can disagree slightly, and readings drift as bulbs and diodes age. Still, a PAR number is far more useful than "it's under the light" when deciding where a colony goes, because identical fixtures produce wildly different intensities depending on mounting height and water depth.

PAR Targets for Acropora

• Established colonies: roughly 250–400 PAR at the coral's surface is the sweet spot. Some sources cite ranges up to 450 PAR for shallow, high-light tanks, while others put a hard floor at 200 PAR for demanding SPS in general — below that, growth stalls and color fades toward brown or green even if the coral survives.
• New frags or freshly imported colonies: start at 150–250 PAR and raise gradually over several weeks.
• Individual variation matters more than one exact number. Color morph, frag origin, and prior lighting history all shift the ideal. Trending consistently in the 250–400 band and watching the coral's response matters more than chasing a specific figure.

Why the Same Fixture Reads Differently Everywhere

PAR falls off steeply with depth. A fixture that reads 500+ PAR just under the water surface can measure under 150 PAR only 20+ inches (51 cm) down. This means placement decisions should be tied to an actual PAR map of your tank — measured with a meter, or estimated from a published PAR chart for your specific fixture, mounting height, and depth — not a blanket "top shelf versus bottom" rule.

If you don't own a meter yet, look for a manufacturer PAR chart that lists readings at multiple depths and mounting heights for your exact fixture. It won't be perfect (output varies with diode or bulb age and water clarity), but it beats guessing from wattage alone.

Practical Placement Zones

Coral status	Placement zone	Target PAR
Established, colored-up Acropora	Upper third of rockwork / upper-mid water column	250–400
New frag or import, week 1–2	Mid-tank, shaded if needed	150–250
New frag or import, week 3–6	Gradually raised toward upper zone	250 → target
Flat/table-form Acropora	Can tolerate closer to intense light at shallower depth	Upper end of range
Thin-branching Acropora	Often needs a slightly more moderate exposure	Mid-to-upper range

Coral shape is a placement cue sources often gloss over: flatter table Acropora generally tolerates sitting closer to an intense light source than thin, delicate branching forms, which can bleach faster at the same PAR reading.

Flow Requirements

Acropora evolved on wave-swept reef crests, so it wants strong, turbulent, multidirectional flow — not one steady jet. Randomized or alternating/pulsing wavemaker patterns beat a single fixed powerhead pointed at the rockwork.

Turnover Targets by Tank Size

There's no single universal GPH number, since needs differ between a frag tank and a mixed SPS display. As a working range:

Tank size	Suggested total turnover (combined pumps)
Nano (under 20 gal / 76 L)	20–25x tank volume/hour
40–75 gal (151–284 L)	25–35x tank volume/hour
75–150 gal (284–568 L)	30–40x tank volume/hour
150+ gal (568+ L)	35–40x+, via multiple pumps rather than one large unit

Judge success by observed effect rather than chasing an exact GPH figure: polyps and tissue should sway gently and continuously, with no visible detritus settling on or near the colony.

Powerhead Placement

• Use at least two powerheads or gyre pumps aimed to cross and collide, creating swirling, randomized flow rather than a straight line.
• Never point a single powerhead directly at a colony's polyps — the constant direct blast causes localized tissue damage and recession on the flow-facing side.
• Avoid dead spots behind large rock formations; a return pump alone (driving flow through a sump) sets baseline turnover, but powerheads create the actual turbulence Acropora needs.
• Larger tanks are better served by multiple smaller powerheads spread around the perimeter than one oversized unit, which tends to leave uneven coverage and stagnant corners.
• Clean impellers on a regular schedule — algae and calcium buildup on the impeller quietly reduces output and can eventually seize the pump, both of which drop your actual in-tank flow below what you think you're running.

You can browse powerhead-compatible SPS corals on FishyHub once your flow and lighting are dialed in.

Spacing and Aquascaping for Placement

Leave several inches of clearance on all sides of an Acropora colony from its neighbors and from the direct light path. Acropora grows outward and branches; overcrowded colonies shade each other and can sting adjacent corals through allelopathy, causing tissue loss where they touch. Shading from a large neighboring colony is one of the top causes of dead, bare lower branches (STN — slow tissue necrosis) in mature aquascapes.

Building Rockwork That Supports Placement

• Create distinct elevation tiers rather than one flat shelf, so you have multiple light and flow zones to work with as colonies grow.
• Leave open channels between rock formations instead of building a solid wall — this lets both light and flow reach every level and avoids stagnant pockets.
• Plan for growth: a small frag needs inches of clearance today, but a mature colony can double or triple in diameter within a year.

Tank Size for Serious Acropora Keeping

Many experienced keepers recommend 75+ gallons (284+ L) for a dedicated Acropora system — not because the coral itself needs a large volume, but because bigger systems hold parameters more stable and give you physical room to separate colonies as they grow without constant fragging or rearranging.

Lighting Technology Notes

Metal Halide

Metal halide historically delivered the most intense, deep-penetrating PAR available, making it the traditional choice for large or deep SPS tanks (24+ inches / 61+ cm) and big frag racks. It produces a natural shimmer effect many keepers like. The tradeoffs are real: high heat output (often requiring fans or a chiller), higher running cost, and bulbs that need periodic replacement as output drops with age.

LED

Modern high-output LED fixtures can match or exceed halide-level PAR with far less heat and much finer control over spectrum and photoperiod. For most US hobbyists setting up in 2026, LED is the practical default — it's more efficient to run and easier to dial in precisely, though a well-run halide system remains a fully viable option for exceptional depth or a frag rack that needs to flood a large footprint.

VHO Fluorescent

VHO (very high output) fluorescent lighting is largely a legacy technology now. It's generally too weak on its own to support demanding Acropora, but still shows up as a supplemental actinic source or in nano setups where intensity demands are lower.

Whichever technology you choose, browse LED lighting options sized to your tank's depth and footprint.

Spectrum and Photoperiod

• Spectrum: a blue-heavy mix — roughly 60–70% blue — with a smaller white and violet/UV component supports both photosynthesis and pigment expression. Too much white or yellow tends to wash out fluorescent coloration.
• Photoperiod: run 8–12 hours of total light with a gradual ramp-up and ramp-down (simulating sunrise/sunset) rather than an instant on/off. Many setups use a shorter 4–6 hour "peak intensity" window within that total.

Lighting a Large or Deep Reef

Deeper and wider tanks need more total output to maintain adequate PAR at the substrate — either additional fixtures, a higher-output unit per fixture, or metal halide for exceptional depth. Corner and edge coverage is often weaker than center coverage under a single fixture, so wide tanks frequently need a supplemental unit at each end rather than one central light stretched to cover everything. Oversized fixtures over a shallow tank need dimming to avoid overshooting the target PAR range entirely.

Full Reef Setup Order

When starting a new system, sequence matters:

1. Cycle the tank fully before adding livestock.
2. Establish flow and skimming first, so water quality and turbulence are stable.
3. Bring lighting online gradually as livestock is added — never blast full-intensity lighting on day one with an empty or lightly stocked tank.

Light Acclimation and Coral Coloration

Any time the PAR at a target spot exceeds what a coral has been living under, acclimate gradually:

• Physically shade the colony with mesh or screen for the first 1–2 weeks, or
• Start it lower in the water column and move it up in stages over 2–4 weeks, increasing intensity by roughly 10–20% at each step.

Gradual increases prevent bleaching and tissue burn, and stable, appropriately dosed spectrum improves pigment expression over weeks to months — color doesn't happen overnight even once placement is correct.

A Practical Acclimation Schedule

Week	Action
1–2	Mid-tank placement or shaded, PAR ~150–200
3	Move up one tier or remove partial shading, PAR ~200–250
4	Move to target zone, PAR ~250–300
5–6	Fine-tune to final target PAR (250–400), monitor coloration

Common Mistakes to Avoid

• Skipping acclimation — placing a new frag straight into a high-PAR zone leads to bleaching or tissue necrosis within days.
• Direct powerhead blast — aiming a single pump straight at a colony causes localized recession on the flow-facing side.
• Overcrowding — colonies placed too close together shade each other and sting on contact, causing tissue loss.
• Ignoring depth-based PAR falloff — a fixture mounted too high or too far above the water can under-deliver PAR even at the top of the rockwork.
• Flow that's too laminar — one-directional flow creates dead spots downstream of rock, where detritus and algae build up around coral bases.
• Guessing from wattage instead of measuring — wattage and PAR aren't linearly related across brands or ages of bulbs and diodes, so an "adequate" fixture on paper can under- or over-deliver in practice.
• Not accounting for aging equipment — halide bulb output drops over its life, and LEDs dim gradually, so a placement that was correct at installation can become insufficient a year later without a periodic PAR re-check.

One more factor tied directly to placement: as an Acropora colony sits in higher light and flow, its growth rate increases, and so does its alkalinity and calcium demand. A placement decision that boosts growth should come with a corresponding look at your dosing routine.

Frequently Asked Questions

How much PAR does Acropora actually need?

Established colonies generally do well in a working range of roughly 250–400 PAR at the coral's surface. Newly acquired frags should start lower, around 150–250 PAR, and be raised gradually over several weeks as they acclimate.

Can I keep Acropora under LED only, with no metal halide?

Yes. Modern high-output LED fixtures reliably reach the PAR levels Acropora needs, with less heat output and far more control over spectrum and photoperiod than halide. Halide remains a viable option for exceptional depth, but it's no longer required.

How much flow does an Acropora tank need?

Aim for strong, randomized, multidirectional flow — generally in the range of 20–40x total tank volume turnover per hour from combined pumps, scaled toward the higher end for larger or deeper tanks. This should come from multiple powerheads working together, not one strong jet.

Where exactly should I place an Acropora frag in my tank?

Put it in the upper third of the rockwork or upper-mid water column, in a spot with measured PAR in your target range and multidirectional flow reaching it. Leave several inches of clearance from neighboring corals on all sides, and acclimate gradually rather than placing it straight into its final position.

Why is my Acropora losing color or turning brown?

This usually points to too little light/PAR for that spot, or unstable/insufficient flow letting nutrients build up around the tissue. Move the colony to better light and flow gradually, and check your water's nutrient levels rather than making a sudden jump in intensity.

Why is my Acropora bleaching or showing tissue recession at the base?

Likely causes are too much light introduced too fast without acclimation, a powerhead blasting the colony directly, or coral-on-coral contact with a neighbor. Shade or reduce intensity temporarily, reposition away from direct flow or contact, and reintroduce full exposure more gradually.

Do I need a PAR meter, or can I estimate from fixture specs?

A meter, or at minimum a reliable PAR chart for your exact fixture at your specific depth and mounting height, is strongly recommended. Guessing from wattage or a generic online chart is unreliable, since output varies by fixture age, water clarity, and even brand-to-brand differences at the same wattage.

Once you've mapped out the light and flow zones in your own tank, you can shop Acropora and other SPS corals on FishyHub and match each colony to the spot that fits its needs.