PAR (Photosynthetically Active Radiation)
PAR measures the amount of light that can be used for photosynthesis. It is the most important parameter for positioning corals and aquatic plants.
Detailed explanation
PAR is measured in micromoles per square metre per second. A high number means a lot of photosynthetic light. A low number means little light.
To understand: under the tropical sun at midday the PAR is around 2000. In a coral reef at 5 metres depth it is around 500. At 20 metres it drops to 100. Corals have adapted to very different levels depending on the depth at which they live.
In the aquarium we reproduce these conditions. SPS corals (Acropora, Montipora, Stylophora) live near the surface in nature and want 250-400 PAR. The LPS (Euphyllia, Favia, Fungia) live deeper and prefer 100-200 PAR. Soft corals (Sinularia, Sarcophyton) tolerate 50-150 PAR.
PAR decreases rapidly with aquarium depth. An LED that produces 500 PAR on the surface can only produce 100 PAR on the bottom of a 60 cm aquarium. This is why the position of the coral matters as much as the power of the ceiling light.
Coralia Lab with the Light & Placement Optimizer calculates the PAR in every point of the aquarium based on the power of the ceiling light, the depth, the turbidity of the water and the distance from the centre of the light beam. It suggests where to place each coral.
For aquatic plants PAR is equally useful in freshwater aquariums. Fussy plants (Hemianthus callitrichoides, Rotala macrandra) require 100+ PAR. Easy plants (Anubias, Java fern) also grow with 30-50 PAR.
Reference values by aquarium type
| Aquarium type | Recommended range |
|---|---|
| Marine (reef) | 50 - 400 umol/m2/sSPS: 250-400. LPS: 100-200. Soft: 50-150 |
| Tropical freshwater | 30 - 150 umol/m2/sDemanding Plants: 100+. Easy Plants: 30-50 |
How it is measured
PAR metre (Apogee MQ-500 is the reference, costs around 400 EUR). Alternatively, the Seneye Reef has an integrated PAR sensor at a more accessible cost. Some smartphone apps try to measure it from the camera but are inaccurate.
What happens when it is too low
Corals fading and stretching towards the light. Spinning plants (long stems with small leaves). Slow growth.
What happens when it is too high
Light bleaching in corals (different from thermal bleaching). The tips of the corals turn white. Plants with burnt or red leaves.
Coralia Lab monitors this parameter with AI diagnostics. Tank Score alerts you when a value moves outside the recommended range for your aquarium.
Try Light & Placement Optimizer for free