PAR Charts

How to Estimate PAR in a Reef Tank Without a Meter

A quantum PAR meter is the accurate way to know reef tank PAR, but a good meter costs more than many people want to spend on a tool they use a few times a year. Below are the methods reefers actually use to get a workable estimate, and an honest account of where each one breaks down.

1. Manufacturer PAR maps and apps

Most reef LED makers publish a PAR map, and several ship an app that predicts PAR from your fixture height and intensity. Red Sea’s ReefBeat, AquaIllumination, and Radion all do some version of this. Match the map to your actual mounting height, find the grid cell over your coral, and read the number.

The catch: these maps are marketing-adjacent, are usually shot in open air or at the water surface, and assume a clean lens and a specific spectrum/intensity. Use them as a starting point, not a measurement.

2. Correcting for depth

Published PAR is typically a surface number. Light is absorbed as it travels down through water, so a coral sitting 30-50 cm below the surface receives clearly less than the map’s top-of-water value. Aquarium guides note the fall-off with depth but generally do not give a clean formula for it, because it depends on your water clarity, tank geometry, and spectrum. The practical takeaway: treat the surface number as a ceiling and assume real delivered PAR at the coral is lower.

3. Sanity-check against coral zones

You do not need an exact number to place a coral well. Cross-check your estimate against the ranges the hobby has converged on:

  • Soft corals (zoanthids, mushrooms, leathers): ~50-150 µmol/m²/s
  • LPS (Euphyllia, Acanthastrea): ~100-250 µmol/m²/s
  • SPS (Acropora, Montipora): ~200-400 µmol/m²/s

If a coral is coloring up and growing where it sits, your estimate is close enough. If it is bleaching, receding, or stretching for light, move it and re-estimate. See the PAR levels for SPS, LPS and soft corals page for the sourced ranges.

4. Borrow or rent a meter once

The highest-value move is a single real PAR map. Many reef stores and clubs rent an Apogee MQ-510 (the common underwater-capable meter) or run a paid PAR-mapping service. One session mapping your tank gives you anchor points that make every later eyeball estimate far more reliable.

Why a phone lux app is a poor substitute underwater

You will see people suggest reading lux with a phone and dividing by a factor. For a reef tank this is weak:

  • Phone sensors are tuned to the human eye (the photopic curve), not the 400-700 nm photosynthetic range, so they misweight the heavy blue spectrum reef lights use.
  • Uncalibrated phone sensors vary widely between models.
  • Putting a phone at the coral means shooting through water and, in practice, through the glass or a housing, all of which distort the reading.

A lux-to-PPFD conversion can give a rough grow-tent ballpark under white LEDs (see estimating PPFD without a quantum meter), but for a blue-heavy reef tank underwater it is closer to a guess than an estimate. For anything where the number matters, borrow a real meter.

Sources

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