Choosing Aquarium Lighting: Fish-Only, Planted, and Reef

Different tanks have very different light needs. Understand the PAR/PUR concept, why plants and corals demand more intensity and specific spectrum than fish-only tanks, and how to set a sensible photoperiod.

Not every aquarium needs the same lighting. A fish-only tank needs light mainly so you can see the fish and to set a day/night rhythm, so its demands are modest. A planted tank needs more, because the plants must photosynthesise to grow. A reef tank asks the most of all, because most corals host symbiotic algae that power the coral with light. In short, light demand climbs from fish-only, to planted, to reef.

PAR and PUR: quantity and quality of light

Photosynthesis is driven by light between roughly 400 and 700 nanometres, known as Photosynthetically Active Radiation (PAR) — this is the light that plants and coral zooxanthellae actually use, and it is a better measure of a light's usefulness than watts or apparent brightness. PUR (Photosynthetically Usable Radiation) is the portion of that PAR whose wavelengths an organism's pigments can actually absorb, which is why spectrum matters and not just raw intensity. In seawater the blue part of the spectrum penetrates deepest, so corals are especially good at using blue light and reef fixtures are weighted toward blue.

How needs differ by tank type

• Fish-only / low-light: modest lighting is enough. Low-light plants such as Anubias, Java fern, mosses and Cryptocoryne tolerate weak light and can be grown without high-intensity fixtures or added CO2.
• Planted: plants need more intensity and a plant-friendly spectrum. Dennerle classifies lighting roughly as 15 lumen per litre (weak), 30 lumen per litre (medium) and 60 lumen per litre (strong); demanding plants and aquascapes need the strong end. More light without matching CO2 and nutrients feeds algae rather than plants.
• Reef: photosynthetic corals rely on light plus their zooxanthellae. Modern reef keeping aims for roughly 350–450 PAR for most corals; high-light SPS corals such as Acropora sit in the brightest zones, while LPS, soft corals and mushrooms do better in lower-light areas.

Photoperiod is the number of hours the light is on. A common recommendation for planted tanks is 8–12 hours a day (about 10 is a good target), and some keepers add a 'midday break' of darkness because plants tolerate it well while algae dislike the interruption. Too long a photoperiod mainly grows algae, so on a new tank start shorter and build up. Modern LED fixtures make this easy: they let you dim, ramp sunrise and sunset, and tune spectrum, and are more efficient than older T5 fluorescent or metal-halide lamps.

Sources: reefbuilders.com , dennerle.com , www.2hraquarist.com