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Aquariums

Aquarium lighting

PAR-at-substrate targets by plant tier. Enter your fixture PAR-at-surface and tank depth, see which plants will thrive.

Aquarium lighting PAR through water
From a PAR meter held at the water line. Manufacturer specs are typically 20-30% inflated; subtract that if you don't have a meter.
From water surface to the substrate. Not the total tank height; subtract the air gap if you don't fill to the rim.
Tannins and dissolved organic carbon attenuate light substantially. Aquasoils that leach tannins start tanks closer to blackwater.
PAR at substrate -- μmol/m²/s
Attenuation --
Plant tier supported --
Example species that work --
CO2 recommendation --
Math: PAR_substrate = PAR_surface × e^(-k × depth). The k values are reasonable averages; real tanks vary with biofilm on glass, debris in the water column, and substrate color (white sand bounces light back up; black substrate eats it).

What this tells you

Aquarium plants get classified into lighting tiers: low light, medium, high, very high. The tiers map to PAR-at-substrate ranges in μmol/m²/s. This calc takes PAR-at-surface (what your fixture puts out at the water line) and a tank depth, applies a rough attenuation factor for water, and tells you which tiers will work at the substrate where plants actually grow.

Substrate PAR matters because that's where roots are and where carpet plants live. A 200 PAR fixture at the surface might deliver only 60 PAR at the substrate of a 45cm deep tank.

The attenuation math

Light through clean aquarium water follows Beer-Lambert roughly:

PAR_substrate = PAR_surface × e^(-k × depth)

Where k is an attenuation coefficient. Clean RO/distilled water with no tannins has k ≈ 0.03 per cm. Tap water with a bit of biofilm and DOC sits around k ≈ 0.05/cm. Tannin-stained blackwater can reach k ≈ 0.10/cm. The calc lets you pick a coefficient based on water clarity.

A 200 PAR fixture at the water surface delivers:

  • 30 cm depth, clean water (k=0.03): 200 × e^(-0.9) = 81 PAR. Medium-light territory.
  • 45 cm depth, normal water (k=0.05): 200 × e^(-2.25) = 21 PAR. Low-light only.
  • 60 cm depth, blackwater (k=0.10): 200 × e^(-6.0) = 0.5 PAR. Almost nothing.

Deep tanks need much more fixture than shallow ones. This is why a 30cm shrimp tank can grow carpet plants with a $30 LED bar but a 60cm "high tech" planted tank costs $200+ for a fixture.

PAR tier ranges

The thresholds used here come from the planted-aquarium community consensus (BarrReport, planted tank guides):

  • Low light: 15-30 PAR at substrate. Anubias, java fern, mosses, crypts. Slow growers that tolerate shade in nature.
  • Medium: 30-80 PAR at substrate. Most rosette plants, swords, easy stems. The bulk of common aquarium plants.
  • High: 80-150 PAR at substrate. Demanding stems (rotala, ludwigia), some carpets. CO2 injection usually required at this PAR.
  • Very high: 150+ PAR at substrate. Hardcore carpets (HC cuba, dwarf hairgrass dense lawns) and red-pigment-dependent species. CO2 always needed; algae management becomes an active job.

Why fixture spec sheets are misleading

Manufacturer-quoted PAR is often measured at the LED face, not at the water surface or below it. A "300 PAR" fixture might measure 200 PAR at typical mounting distance (10cm above water) and 60 at the substrate of a 40cm tank.

If you have a PAR meter, measure at substrate yourself. If not, subtract 30% from the manufacturer's spec for honest at-surface PAR.

Further reading