Lighting basics: PAR, spectrum, and photoperiod without the marketing

Aquarium lighting marketing is full of big numbers. Here is what actually matters, minus the hype.

Three things that matter:

1. Intensity (PAR) — how much usable light reaches your plants or corals. Low-light plants want modest PAR; high-light plants and corals need much more — and more light without matching CO2/nutrients just grows algae.
2. Spectrum — the color mix. Plants look best around 6500K (daylight); reef corals are lit heavier on blue. "Full spectrum" covers most needs.
3. Photoperiod — how long it is on. Most planted tanks do well on 6–8 hours; pushing 10–12 is a classic cause of algae, not lush growth. Use a timer.

For a fish-only tank, lighting is purely aesthetic — pick what looks good and keep the photoperiod modest to limit algae.

For planted or reef tanks, light is one leg of a tripod with nutrients and (for plants) CO2. Crank one without the others and algae wins.

Your turn: what are you lighting — fish-only, planted, or reef — and what fixture and photoperiod are you running? Algae creeping in? Post a photo.