Aquarium lighting: lamps and parameters

The modern aquarium industry is dominated by LED lighting. Old T5/T8 fluorescents have moved to the background. The "which lamp do I buy?" question is now more complex: color temperature, spectrum, power, PAR.

What matters for fish

Fish need a "day" and a "night" — 8–12 hours of light and 12+ hours of darkness. Exact color temperature isn't critical — in the wild they live perfectly well under cloudy skies and bright sun alike.

Color temperature affects perception of color: 6500K (neutral white) looks most natural, 7500K+ is colder, 4000–5000K is warmer with a reddish cast.

What matters for plants

Here the technical specs matter: spectrum and intensity determine whether plants will grow.

Kelvin (K)

Optimal for most aquarium plants — 6500–7000K. It's close to midday sunlight. Plants can use any light, but 6500K gives the most "green" and natural visual result.

Lumens and PAR

Lumens is brightness as perceived by the human eye. For plants, PAR (photosynthetically active radiation) matters more — a measure of photons in the spectrum chlorophyll actually absorbs.

For low-tech plants: 20–30 μmol PAR over the substrate. For high-tech (with CO₂): 50–80 μmol.

Watts per liter (an outdated metric)

The old 0.5–1 W/L rule worked for fluorescent tubes. With LED it's useless — a modern 30 W LED fixture delivers more PAR than a 60 W T8.

Plant categories by light demand

• Low light (15–20 μmol): anubias, Java fern, cryptocoryne, mosses. Cover-glass LED of 4–6 W is enough.

• Medium light (25–35 μmol): vallisneria, echinodorus, hygrophila, ludwigia. A full-size LED fixture.

• High light (50+ μmol): glossostigma, rotala, Monte Carlo. Requires CO₂ and fertilization.

Photoperiod length

8–10 hours is optimal. Less than that — plants are underfed. More than 12 hours — algae blooms.

A midday "siesta" period — disputed technique. Benefits not proven; a single 8-hour timer slot is simpler.

Signs of problems

• Black spots on old leaves — light is too bright.

• Plants stretch up and bare their bases — not enough light.

• Green spots on glass — normal, otocinclus/nerite snails eat it.

• Black beard algae (BBA) — too much light + CO₂/nitrate imbalance.

Before buying a light, decide which plants you want to keep. Anubias and Java fern will grow under a kitchen bulb. Glossostigma demands a $200 professional LED.