PAR Meter Reef Tank Guide: Measuring Coral Light

You cannot dial reef lighting by eye — what looks bright to the human retina bears little relation to what corals photosynthesise under. A PAR meter is the difference between guessing and knowing, between frags that colour up in six weeks and frags that pale steadily until they RTN. This par meter reef tank guide from Gensou Aquascaping at 5 Everton Park explains what the meter actually measures, which models to trust on SG waters, and how to run the measurement correctly so the numbers mean something. Reef lighting stops being magic the moment you can quantify it.

What PAR Measures

PAR stands for photosynthetically active radiation — the 400-700 nm light band that drives photosynthesis. A PAR meter integrates intensity across this band and reports a single number in μmol photons per square metre per second (PPFD). High-PAR zones suit SPS, mid-range suits LPS, low-PAR suits softies. The reading tells you where to place each coral — not a vanity metric but the single most useful dataset for reef husbandry.

Quantum Sensors vs Quantum-Corrected

Standard Apogee quantum sensors (MQ-500, MQ-510) are calibrated to natural sunlight spectrum. Reef LEDs with spiked royal blue output read 15-30% low on these sensors because the sensor rolls off below 450 nm. Quantum-corrected sensors (Apogee SQ-520 and similar) flatten the response across reef LED spectrums for accurate readings. For Singapore reefers running mostly LED fixtures, the corrected model is worth the SGD 100-150 premium — the uncorrected model lies to you.

Apogee Models in SG

Apogee MQ-510 (SGD 550) is the entry reef-grade underwater model with quantum correction for electric light sources. Apogee MQ-210 (SGD 480) runs a shorter immersion cable for nano work. Seneye Reef (SGD 320) measures PAR plus NH3, pH and lux via smart USB device — convenient but less accurate than Apogee at the high PAR end. Budget route: rent a PAR meter from a LFS for SGD 30 per weekend rather than buying — reasonable if you only measure twice a year.

Setting Up the Measurement

Turn off all powerheads and return flow before measuring — moving water surface refracts light unpredictably and throws readings by 10-20%. Submerge the sensor to substrate level, position flat-facing upward, and wait 5 seconds for the reading to stabilise. Record the number. Move the sensor to mid-scape height, then top of rockwork. Repeat at four points across the tank length — light drops dramatically away from centre under point-source LEDs.

Reading the Map

Expect high-PAR zones directly under the light array at 350-500 PPFD, dropping to 100-200 at mid-height and 50-150 at substrate. A well-designed reef places SPS on high-PAR rockwork peaks, LPS on mid-level ledges, and softies at the base where PAR sits 75-120. Corals that never grow despite good parameters are usually misplaced in the PAR map — move before replacing.

Target PAR for Coral Types

Zoanthids, mushrooms and GSP thrive at 50-100 PAR. Euphyllia (hammer, torch, frogspawn) sit at 75-150 PAR. Acans, chalices and most LPS prefer 100-200 PAR. Montipora digitata and cap variants want 150-300 PAR. Acropora spp demand 300-450 PAR in the top third of the scape. Tridacna clams want 250-400 PAR. These are tolerance ranges — acclimate new frags at half the target for two weeks before final placement.

Light Ramping and Acclimation

A PAR meter transforms coral acclimation from guesswork to process. New frag coming from an LPS display running 80 PAR cannot go straight into your 250 PAR SPS zone. Place it at 80-100 PAR for two weeks, move to 150 PAR for another two weeks, then target placement. Bleaching, paling and brown-out nearly always trace to PAR jumps rather than chemistry when parameters hold stable.

Lamp Ageing and PAR Drift

LED fixtures lose 10-20% PAR over 3-4 years even while looking the same. T5 bulbs drop 20-30% in 12 months. Measuring PAR annually catches this drift before corals pale. Fixtures programmed at 80% intensity three years ago might need bumping to 90% to maintain the same delivered PAR. Without measurement you cannot catch this — you only notice when corals recede and by then you are months behind.

SG Reef Use Cases

HDB HDB-scale reef tanks typically run closed or semi-closed top to reduce chiller load. Canopies and glass lids can absorb 15-25% of incident PAR before it hits the water. Measure under whatever cover configuration you actually run — not with the lid off. Keepers sharing Apogee meters via local reef Facebook groups (Reefkeepers Singapore, Gensou aquascaping forum) save the SGD 550 purchase price if you only need the map once.

When You Might Not Need One

Pure softie and mushroom tanks running entry-level LED fixtures at 60 cm or less rarely benefit from PAR measurement — the output is naturally in the 50-150 range across the scape and corals self-acclimate. SPS and mixed-reef keepers cannot reasonably skip PAR measurement; the cost of one bleached SGD 250 acro outstrips the SGD 550 meter on the first mistake. For the middle ground of LPS-heavy mixed reefs, rent a meter twice a year rather than buying.

Related Reading

• Reef Tank Lighting Beginner Guide
• First Coral Beginner Complete Guide
• Hardy Soft Corals Beginner Guide
• Reef Tank Acclimation Complete Guide
• Reef Tank Parameters Beginner Guide