How many hours of light
Plants in a new tank need time to root before they can use much light, while algae can use it straight away. Tropica’s growing-in advice is 6 hours a day for the first 2–3 weeks, then gradually more. Established planted tanks commonly run 8 hours, and some with CO₂ and fast-growing plants run up to 10.
A timer matters more than the exact number. Fish and plants do best with a steady day length, and a consistent schedule avoids accidental 14-hour days.
| Situation | Hours per day |
|---|---|
| New planted tank, first 2–3 weeks | 6 |
| Settling in (weeks 3–6) | 6–7, rising |
| Established planted tank | 8 |
| CO₂, demanding plants, no algae | 8–10 |
| No live plants (viewing only) | 8 or less |
| Algae problem | Cut 1–2 hours |
How bright: lumens per litre
Tropica recommends comparing lights by lumens (light output) rather than watts, because LEDs, T5 and T8 tubes turn electricity into light very differently. Its guidance, based on years of T5 lighting: Easy plants 10–20 lm/L, Medium 20–40 lm/L, Advanced over 40 lm/L.
Tropica’s own example: a 54 L starter tank with one 15 W T8 tube gets about 900 lumens (17 lm/L, easy plants). A 24 W T5 tube gives 1,600 lumens (30 lm/L, medium plants).
| Water volume | Easy plants | Medium plants | Advanced plants |
|---|---|---|---|
| 30 L (8 US gal) | 300–600 lm | 600–1,200 lm | Over 1,200 lm |
| 54 L (14 US gal) | 540–1,080 lm | 1,080–2,160 lm | Over 2,160 lm |
| 100 L (26 US gal) | 1,000–2,000 lm | 2,000–4,000 lm | Over 4,000 lm |
| 200 L (53 US gal) | 2,000–4,000 lm | 4,000–8,000 lm | Over 8,000 lm |
| 300 L (79 US gal) | 3,000–6,000 lm | 6,000–12,000 lm | Over 12,000 lm |
Light and algae
- Too much light for the plants feeds algae. Strong light needs matching CO₂ and nutrients; without them, plants cannot keep up.
- Green water: Tropica suggests a blackout of 2–3 days, then frequent water changes. A UV sterilizer also clears it.
- Watch for daylight. A tank near a sunny window can get hours of extra light the timer does not control.
- Colour: Tropica recommends 5,500–6,500 K for freshwater, close to daylight.
