Lighting: what are the different types?

Why your choice of light actually matters

Light is one of the three variables - alongside water parameters and feeding - that most directly controls what your tank looks like six months in. Get it right and you have healthy fish, vibrant colours, and (if you want them) thriving plants. Get it wrong and you get a tank coated in brown or green algae, bleached-out fish, and plants that slowly melt.

Fish-only tanks need enough light to replicate a natural day/night rhythm and let you actually see the animals. Planted tanks need specific wavelengths at sufficient intensity for photosynthesis to outpace algae. These are different jobs, and the light you choose should match whichever one you're doing.

LED lighting

LEDs are what most people should buy today. The technology has matured enough that even budget units - the kind bundled with starter tanks - outperform the fluorescent lights they replaced. A decent LED fixture draws a quarter of the power of an equivalent T8 setup, produces far less heat, and the diodes themselves last 30,000-50,000 hours before meaningful degradation.

For planted tanks, the key spec is PAR (photosynthetically active radiation), measured in µmol/m²/s at the substrate level. This is the number that tells you whether your light can actually grow plants, and most manufacturers have started publishing it - look for it before you buy.

What to look for in an LED fixture

For a fish-only tank, almost any full-spectrum or daylight LED (5,000K-7,000K colour temperature) will do the job. The light that came with your aquarium is probably fine.

For a planted tank, you want a fixture that publishes PAR data and hits at least 50 µmol/m²/s at the substrate for low-light plants (Java fern, Anubias, most mosses) or 100-150+ for stem plants and carpeting species. Brands like Fluval, Chihiros, and the mid-range Aquael units hit this comfortably without spending a fortune. High-end options from Twinstar or the AI Blade series are excellent but unnecessary for most planted setups.

Many modern LEDs offer programmable schedules - sunrise/sunset ramps, cloud cover simulation, intensity control via an app. These are genuinely useful for managing algae, not just aesthetics. Being able to run 60% intensity for the first two weeks of a new setup while your plants establish is a real practical advantage.

Fluorescent lighting: T8 and T12

T8 and T12 tubes were the standard for aquarium lighting for decades, and they're still sold widely. The "T" number refers to tube diameter in eighths of an inch - a T8 is one inch across, a T12 is 1.5 inches. T8 is more efficient and has largely replaced T12 in new equipment.

For a basic fish-only tank, a good T8 daylight tube (6,500K) does the job. The light is even across the tank length, colour rendering is natural, and replacement tubes are cheap and available everywhere. If you already have a fluorescent fixture and your tank has no live plants (or only very low-light species like Java fern and Anubias), there's no urgent reason to upgrade.

The real limitation of T8 is intensity. Even with multiple tubes, you'll struggle to hit PAR values above 50-80 µmol/m²/s at the substrate of a standard-depth tank (45cm/18 inches). That's low-light territory only. If you want to grow stem plants, rotala, or anything that needs moderate to high light, T8 fluorescent isn't the right tool.

T5 and T5 HO fluorescent

T5 tubes are thinner (5/8 inch diameter) and, in high-output (HO) form, significantly brighter than T8. A twin-tube T5 HO fixture over a medium-depth tank can realistically hit 80-120 µmol/m²/s PAR at the substrate - enough for most medium-light planted tanks and many high-light species if you run three or four tubes.

One genuine advantage of T5 HO is the ability to mix tube types in the same fixture - one 6,700K daylight tube for overall brightness, one 8,000K or "plant" spectrum tube for the red-blue wavelengths plants use most. This was a common approach in the planted tank hobby before quality LEDs became affordable, and it works well.

The honest reason to choose T5 HO today is that you already own the fixture. If you're buying new, a mid-range LED will outperform T5 HO in efficiency, adjustability, and lifespan. T5 HO bulbs need replacing every 10-12 months in a planted setup, and good tubes aren't cheap. Over three to four years, you'll spend more on tubes than you would have on a better LED fixture upfront.

Spectrum: what "full spectrum" actually means

Light spectrum is the distribution of wavelengths a fixture produces. Plants use primarily blue (430-450nm) and red (640-680nm) wavelengths for photosynthesis. Green light (around 550nm) is less efficiently used, which is why plants reflect it - that's why they look green.

For fish-only tanks, spectrum is mostly about aesthetics. A 6,500K-7,000K "daylight" white makes colours look natural and vibrant. Heavily blue-shifted lights (like reef actinics) make freshwater fish look washed out and unnatural, so avoid them for freshwater setups.

For planted tanks, you want a fixture that covers both the blue and red peaks. Most LEDs marketed as "plant" or "full spectrum" do this reasonably well. The colour temperature you perceive (the warm/cool tone of the light) is less important than whether the fixture actually emits in those wavelengths - which comes back to checking manufacturer PAR and spectrum data rather than trusting marketing copy.

Intensity: matching light to your plants

PAR is the number that tells you whether your plants will grow. Here's a practical breakdown:

• 0-20 µmol/m²/s: Too low for most plants. Fine for fish-only.
• 20-50 µmol/m²/s: Low light. Suitable for Anubias, Java fern, most mosses, and crypts.
• 50-150 µmol/m²/s: Medium light. Most stem plants, Amazon swords, Vallisneria, and the majority of what beginners want to grow.
• 150+ µmol/m²/s: High light. Carpeting plants (Hemianthus callitrichoides, Eleocharis), demanding stem plants, and competition-style aquascapes. Requires CO2 injection to avoid algae.

Most people running a "planted" tank without CO2 injection are actually better served staying in the 50-100 µmol/m²/s range. Higher intensity without CO2 just accelerates algae growth - the plants can't use the extra light productively without the carbon to support faster growth.

Photoperiod: how long to run your lights

Most people run their lights too long. The instinct is that more light hours means more plant growth - it doesn't. Plants hit light saturation quickly; extra hours mostly feed algae.

6-8 hours per day is the target for planted tanks. Fish-only tanks can run 8-10 hours for a natural day/night cycle. Going beyond 10 hours is almost always a mistake, and if you're fighting persistent algae, cutting your photoperiod to 6 hours is one of the first and most effective interventions.

Use a timer - the mechanical plug-in type costs almost nothing and removes the human inconsistency that throws off the tank's rhythm. Many LED controllers have this built in. Consistency matters: your fish and plants respond to a regular cycle, and randomly long or short days cause stress and growth disruption.

What most guides don't cover: light and tank depth

PAR values drop significantly with water depth. A fixture that delivers 120 µmol/m²/s at 10cm below the surface might deliver only 40-50 µmol/m²/s at 40cm. For tall tanks (over 50cm/20 inches), you either need a more powerful fixture or you accept that only plants in the upper half of the water column will thrive.

This is why "watts per gallon" rules - still repeated in older guides - are meaningless. A tank's footprint, depth, and water clarity all affect how much useful light reaches your plants. PAR measured at the substrate is the only number that counts.

Surface agitation and floating plants also cut light penetration. A heavy surface ripple from a powerhead can reduce PAR at depth by 10-20%. Floating plants like duckweed or frogbit are extreme - they can block 70-80% of light to anything below them within days.