PAR by Tank Depth Chart Guide: Targets per Plant Tier
Lighting fixture wattage means nothing without knowing how many photons actually reach your substrate. PAR by tank depth is the only measurement that matters for plant growth, and the same fixture that delivers 150 µmol on a 30cm-tall tank delivers 60 µmol on a 50cm-tall one. This guide from Gensou Aquascaping at 5 Everton Park provides PAR target ranges per plant tier and a depth-by-depth chart calibrated for popular fixtures available in Singapore.
What PAR Actually Measures
PAR (Photosynthetically Active Radiation) measures photon flux density between 400-700 nm in µmol/m²/s — the exact wavelengths plants use for photosynthesis. Lumens, lux, and watts per litre are useless for plant growth because they weight wavelengths plants ignore. A SGD 450 PAR meter reads what plants see; a SGD 30 lux meter does not.
Plant Tier PAR Targets at Substrate
Low-light tier (Anubias, Java Fern, Cryptocoryne, mosses): 30-50 µmol at substrate. Mid-light tier (most stems, Bucephalandra, Vallisneria): 50-100 µmol. High-light tier (red Rotalas, AR Mini, Eriocaulon, dwarf carpet plants): 100-150 µmol. Extreme tier (HC Cuba carpet under heavy CO2): 150-250 µmol. Above 250 µmol you are pushing into algae-blooming territory unless CO2 and dosing are perfect.
Depth Halving Rule
PAR drops roughly 50 per cent for every 25 cm of water depth, ignoring scattering. A fixture reading 200 µmol at the surface gives 100 µmol at 25 cm depth and 50 µmol at 50 cm. Singapore-popular ADA 60P (36 cm tall internal) needs roughly 70 per cent of surface PAR at substrate; a 90H (50 cm tall) needs 100+ per cent stronger surface PAR to deliver the same numbers.
Chart by Common Tank Depths
30 cm depth: surface 60-80 µmol gives substrate 50 µmol (low-light); surface 150 µmol gives substrate 100 µmol (high-light). 40 cm depth: surface 80-100 µmol gives 50 µmol substrate; surface 200 µmol gives 100 µmol substrate. 50 cm depth: surface 100-150 µmol gives 50 µmol substrate; surface 280 µmol gives 100 µmol substrate. 60 cm depth: surface 200 µmol gives 50 µmol substrate; surface 400 µmol approaches the upper limit of practical fixtures.
Fixture PAR at 30cm Mounting Distance
ADA Solar RGB at 30 cm mount delivers around 250 µmol surface, 80-100 µmol at 30 cm depth. Chihiros WRGB II 60: 200 µmol surface, 70 µmol at 30 cm depth. Twinstar 600B at 30 cm: 180 µmol surface, 60 µmol at 30 cm depth. Week Aqua Z series at 30 cm: 220 µmol surface, 75 µmol at 30 cm depth. The aquarium LED range at Gensou stocks fixtures across this PAR spectrum.
Substrate Reflection and Hardscape Shading
Black substrate absorbs 80 per cent of incident PAR; white substrate reflects 60 per cent back upward, effectively boosting substrate-level PAR by 20-30 per cent. Hardscape shading is the killer most keepers ignore — wood and rock can shade 40 per cent of substrate area. Carpet plants under shaded zones starve regardless of fixture spec; design hardscape with light penetration in mind, or accept moss in shaded areas instead of carpets.
Photoperiod Trade-Off
Lower PAR with longer photoperiod can match higher PAR with shorter — within limits. 50 µmol for 10 hours integrates roughly the same as 75 µmol for 7 hours. Plants do not care about peak intensity but total daily photons. The catch: algae also responds to photoperiod, and 10 hours is the practical algae trigger ceiling for most setups regardless of intensity.
Singapore Practical Notes
Open-top rimless tanks lose 5-10 per cent PAR to surface refraction and water-droplet glare. Tropical 28-30°C operating temperatures slightly reduce LED output (semiconductors lose efficiency at heat) — fan-cooled fixtures hold spec better than passive heatsinks in Singapore HDB conditions. A low-iron tank transmits 5-7 per cent more PAR than standard glass; meaningful on 60cm-tall setups.
Verifying With a PAR Meter
Borrow a Seneye, Apogee MQ-510, or budget Chinese PAR meter (SGD 200-1500) and measure at four points: substrate centre, substrate corner, mid-water under hardscape, and water surface centre. Note the delta between centre and corner — most fixtures fall off 30-40 per cent at corners on standard 60-90cm tanks. Measure at hour 3 of photoperiod, fixture at full output, no LED ramping artefacts. Repeat annually as LEDs degrade 10-15 per cent over three years.
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