SPS - Small Polyp Stony coral - is shorthand for the demanding, high-light, high-flow, parameter-sensitive end of the saltwater reef hobby. The dominant SPS genera are Acropora, Montipora, Stylophora, Pocillopora, Seriatopora, Hydnophora, and Echinopora. They share calcium-skeleton building, photosynthetic symbiosis with zooxanthellae, and a near-religious sensitivity to alkalinity stability.
The hobby separates into two phases. Phase one is keeping SPS alive - hardier species like green slimer, Stylophora pink, and standard Montipora cap will tolerate moderate parameter excursions and beginner mistakes. Phase two is keeping the named-cultivar economy alive - Pink Lemonade, Walt Disney Tenuis, Tyree Purple Monster - which demands the kind of precision that turns reefkeeping into a discipline.
This guide is for phase two. If your tank is less than 12 months old, save this for the future and master the basics first. If you're ready: read.
Most beginner SPS articles list parameters: alkalinity 8-9 dKH, calcium 420-450, magnesium 1300-1400, salinity 1.025-1.026. That's correct but useless. The framework that actually matters is stability over absolute values. SPS tolerates 7.5 dKH and 9.5 dKH; it does not tolerate swinging between 7.5 and 9.5 in 48 hours.
Alkalinity (KH) measures the bicarbonate buffering of seawater. SPS deposits calcium carbonate by combining calcium ions with carbonate ions; alkalinity is the pool of available carbonate. As coral grows, alkalinity is consumed. As pH swings (CO2 from ambient air), alkalinity reads can fluctuate.
The hard rule: alkalinity should not move more than 0.3 dKH per day, and should sit between 7.5 and 9.0 dKH. The exact target within that band is less important than picking one and holding it. Tanks running 7.8 dKH stable produce better SPS coloration than tanks running 8.5 dKH with 0.5 dKH variance.
Alkalinity above 9.0 in low-nutrient systems triggers tip burn and tissue recession - the so-called "alk burn" diagnostic. Alkalinity below 7.0 starves coral skeleton building and produces pale, slow-growth corals. Both extremes are correctable by ramping back to 8.0-8.5 over 7-10 days.
Calcium 420-450 ppm, magnesium 1300-1400 ppm. Calcium is consumed in lockstep with alkalinity (1:1 molar ratio in skeleton building). Magnesium is the "third leg" - it prevents calcium and alkalinity from precipitating out of solution as cloudy water. Magnesium deficiency manifests as alkalinity that won't hold steady despite consistent dosing.
Test calcium weekly; once it's stable, biweekly is enough. Magnesium should be tested monthly - it depletes slowly and resists swings unless something is consuming it (tridacna clams, certain calcareous algae).
The biggest mistake new SPS keepers make: trying to run "ultra-low nutrient" systems. NO3 below 1 ppm and PO4 below 0.02 ppm produce pale, photosynthesizing-but-not-feeding corals that bleach under stress and lose color. Modern SPS keeping accepts that SPS need food: light feeding, modest nitrate (2-5 ppm), and detectable phosphate (0.04-0.10 ppm) produce richer color expression and better growth.
The nitrate-to-phosphate ratio matters. Redfield ratio (16:1 N:P by atoms) translates to roughly 100:1 N:P by weight. Reef tanks tend to drift below Redfield (more P than N), causing dinoflagellate and cyano blooms. If your phosphate runs higher than 0.15 ppm with nitrate under 2 ppm, dose nitrate (NeoNitro, Brightwell potassium nitrate) to bring the ratio in line.
1.025-1.026 SG measured with a refractometer calibrated against 35 ppt calibration solution. Hydrometers are inadequate for SPS keeping - they read consistently 0.001-0.002 SG off and that delta is enough to stress coral over months.
76-78F, stable. Daily swing under 1F. Heaters with controllers (Inkbird, Apex) protect against runaway events. Chillers are required in summer for systems above 80F when LED retrofit hasn't reduced ambient heat load.
Modern reef LED has won the lighting wars. Metal halide and T5 still produce excellent coral coloration but at 4-6x the operating cost and significant heat output. The current LED options:
SPS PAR requirements are species-specific:
Buy a PAR meter (Apogee MQ-510 is the standard at $500). Map your tank zones. Place coral by PAR, not by visual estimate.
Coloration in SPS is a function of fluorescent protein expression, which is triggered by specific blue and UV wavelengths. The key bands:
Heavy-blue spectra (Radion AB+ profile, Orphek 24K-equivalent) produce maximum color expression but make the tank visually dim to human eyes. Most SPS keepers run blue-heavy during display hours and white-pop during peak photo hours.
SPS evolved on reef crests where wave action produces 30,000+ gph effective flow. Aquariums replicate a fraction of that, but the principle holds: SPS need turbulent, multidirectional, dead-spot-free flow. Insufficient flow produces detritus settling on coral tissue, slow growth, algae attachment to bases, and poor polyp extension.
Sizing rule: aim for 30-50x tank volume per hour total flow capacity. A 75-gallon SPS tank wants 2,500-3,500 gph capability split across 2-3 pumps to avoid laminar zones.
SPS coral consumes alkalinity, calcium, and magnesium continuously. Without dosing, parameters drop. Three approaches:
Separate alkalinity and calcium solutions, dosed independently via dosing pump. Bulk Reef Supply (BRS) sells the standard kit. Easy to control, easy to test, easy to adjust. Modern dosing pumps (Kamoer X4 PLUS, Apex DOS, Hydros XP) are programmable and reliable.
CO2 dissolves CaCO3 media (aragonite or coarse coral skeleton), producing calcium and alkalinity in 1:1 ratio. Hands-off once dialed but requires CO2 cylinder, regulator, pH-controlled solenoid, and weekly tuning. Suited to large SPS systems with predictable consumption.
A single solution that delivers calcium, alkalinity, magnesium, and trace elements in marine-balanced ratio. Simplifies dosing (one liquid, one pump) at higher per-gallon cost. Excellent for medium SPS systems where simplicity matters more than scale economics.
Three solutions covering all elements coral consumes, dosed in fixed ratios based on system size. Pairs with quarterly Triton ICP testing to dial trace element delivery. Premium approach with strong following among UK and EU reefers.
SPS communicates its problems via specific symptoms. Train yourself to read them.
Cause: nutrient starvation. Nitrate below 1, phosphate below 0.02, or both. Fix: dose nitrate, increase feeding, accept slightly higher nutrients (3-5 NO3, 0.05-0.10 PO4).
Cause: nutrient excess. Nitrate above 15, phosphate above 0.20. Fix: more aggressive water changes, GFO for phosphate, refugium expansion, carbon dosing.
Cause: alkalinity swing. Recent dosing change, recent water change with mismatched alk, or test kit error. Fix: measure alk three times with two different kits to verify, adjust slowly, expect 6-week recovery.
Cause: alkalinity above 9.5 in low-nutrient system, OR PAR shock from lighting upgrade, OR new coral acclimating. Fix: lower alk to 8.0, reduce light if recent upgrade, allow 2-week acclimation.
Cause: extreme parameter swing, infection at frag wound, AEFW at base of branch. Fix: dip in CoralRx or Bayer immediately, frag above necrosis line, glue to fresh plug, isolate to frag tank for observation.
Cause: chronic parameter instability. Fix: full parameter audit, switch to a stable dosing approach, accept 8-week recovery.
Cause: pest (red bugs, AEFW), water quality issue, or chemical warfare from neighboring coral. Fix: visual pest inspection at night under blue light, water change, isolate suspect coral.
SPS pests fall into several categories. The most damaging:
A 75-gallon SPS-capable reef costs $4,000-7,000 to build with quality equipment, $80-150/month to operate, and $1,500-4,000/year in coral acquisition for active collectors. The named-cultivar economy can push that significantly higher - a single Walt Disney Tenuis frag is $400-800; a Pink Lemonade colony piece is $400-900. Serious SPS reefers hit $20,000-50,000 of accumulated coral value within 2-3 years.
Fast Aquatics' vendor scorecards and verified-lineage badges become especially valuable in this segment. Generic "Pink Lemonade" frag at $80 versus Tyree-traced Pink Lemonade at $180 - the price delta only justifies itself with documented lineage.
Beyond this guide, the SPS keeper's required reading:
SPS keeping rewards discipline over wealth. A reefer running stable parameters with cheap equipment will out-grow a reefer running expensive equipment with sloppy parameters. Pick your alkalinity number and hold it. Pick your nutrient targets and keep them. Pick your light intensity and don't move it. The cultivar economy waits for keepers who can do this for 12+ months without flinching.