The Apogee Quantum Meter (model MQ-200) is ideally suited for use in aquariums. The sensor head is potted solid and completely sealed. There is no hollow cavity for water to eventually penetrate and cause measurement errors. The blue lens improves the spectral response to more accurately measure all wavelengths of light. The logging capability of the meter allows you to monitor light levels on a half hour basis for 99 measurements and stores a daily total for over three months.
Attach sensor with zip ties to a long piece of pvc pipe, to conveniently move around your aquarium, when testing for various PAR levels.
Sufficient lighting is vital for growing healthy coral. There are two critical components to adequate lighting for aquariums, intensity and spectra. Zooxanthellae is the algae that lives on the coral and through photosynthesis, provides glucose, glycerol, amino acids and oxygen for the coral. Light (or photons) in the wavelengths of 400 to 700 nanometers is responsible for photosynthesis and is referred to as Photosynthetically Active Radiation (PAR) or Photosynthetic Photon Flux (PPF). This range is also referred to as Quantum.
The appropriate light intensity for a saltwater coral reef aquarium would be about 20 to 25 μmol m-2 s-1 for every gallon of water, for fish tanks 24" deep with a minimum of 10 μmol m-2 s-1 per gallon. For example, for a 25 gallon aquarium you would need approximately 500 μmol m-2 s-1.