Intensity at the leaf is what actually counts
Plants respond to the light that reaches their leaves, not the light that leaves the fixture. Water absorbs light quickly, so intensity falls off with depth: a lamp that grows plants happily in a shallow tank may leave the bottom of a deep tank in effective twilight. This is why experienced planted-tank keepers talk about light reaching the substrate rather than quoting the wattage printed on the box. Foreground carpeting plants sit furthest from the lamp and are the first to fail when intensity is short, stretching upward or simply melting away, while tall stems near the surface manage on almost anything. If you remember one principle, make it this: judge your light at the lowest leaf you expect to grow.
Colour: full spectrum, daylight white
Plants photosynthesise most efficiently under light rich in the red and blue parts of the spectrum, and a good full-spectrum aquarium light builds both into a clean daylight white, usually rated around 6,500K. That daylight rating matters practically, not just biologically: it renders greens naturally, so you can actually see whether a leaf is healthy, yellowing or coated in algae. The coloured novelty modes on some fixtures — deep blue evenings, purple glows — are for human entertainment and do little growing. A plant light should look like a bright window at midday, because that is roughly what it is imitating.
Hours: the photoperiod plants can settle into
Most planted tanks grow best on six to eight hours of light a day, rising toward ten in a mature, densely planted setup where the plant mass can use it. More hours is not more growth: plants photosynthesise on a rhythm and effectively close down after their day is done, while algae keep exploiting every extra hour you give them. This is why a timer is not an accessory but the heart of the lighting setup — the same start and finish every day, unaffected by weekends, late nights or forgotten switches. A brand-new planted tank deserves the short end of the range, because its plants are still rooting and algae find an under-planted tank easy pickings.
Matching light to the plants you actually keep
Light ambition should match plant ambition. The easy, slow species — anubias, java fern, most cryptocoryne, mosses — grow contentedly in low to moderate light and ask for nothing else special. Fast stem plants want a step up in brightness to stay bushy rather than leggy. The true high-light plants, the low carpets and red species, only hold their form when strong light is paired with injected carbon dioxide and regular fertilizer; give them the light without the other two and you have built an algae farm with excellent illumination. Choosing plants for the light you own is always cheaper and calmer than upgrading the light for plants you impulse-bought.
Reading the feedback your tank gives you
The tank reports on its lighting continuously. Stems that grow pale, thin and stretched toward the surface are asking for more intensity. Leaves that yellow low down while the tops thrive are being shaded out. A light dusting of algae on the glass in an otherwise lush tank is normal life; a green bloom coating everything within days usually means the photoperiod is too long or the light too strong for the amount of plant growth and fertilizer in the system. Adjust one variable at a time — hours first, because it costs nothing — and give each change a couple of weeks before judging it. When a new fixture or plant list is on the table, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library before you spend anything.