Balancing light, CO₂ and fertiliser in a planted tank

In a planted tank, light sets how fast plants try to grow, while CO₂ and fertiliser supply what that growth needs. The three must match: the scarcest one limits growth and whatever plants cannot use feeds algae. Choose plants for your light, give medium to demanding plants 10–30 mg/L CO₂, and start new tanks at about 6 hours of light.

Light, easy plants10–20 lumen per litre (≈0.25–0.5 W/L)
Light, demanding plantsOver 40 lumen per litre (>1 W/L)
CO₂ target10–15 mg/L medium, 15–30 mg/L demanding
Drop checker4 °dKH solution; green = about right
Photoperiod6 h at start, then 8–10 h
Colour temperature5,500–6,500 K
CO₂ timingOn 1–2 h before lights, off ~1 h before lights-out
First fertiliserAfter 3–4 weeks, below the label dose

Why do light, CO₂ and nutrients have to be balanced?

Because plants can only grow as fast as their scarcest resource allows. This is the law of the minimum, credited to Carl Sprengel and popularised by Justus von Liebig around 1840. Light is the accelerator: more light raises the plants' demand for CO₂ and nutrients, so both have to rise with it, and the other way round.

In most tanks CO₂ is the first thing to run short; Tropica calls added CO₂ the most important plant fertiliser. After that come nitrogen, phosphorus, potassium, iron and trace elements. When one is missing, plants stall or melt, their dying leaves release nutrients, and algae use both those nutrients and the light the plants could not.

What is the difference between high-tech and low-tech planted tanks?

A high-tech tank runs strong light, injected CO₂ and full fertilisation: plants grow fast and look lush, but it needs more work. A low-tech tank uses modest light, no CO₂ and a nutrient soil or light dosing: growth is slow, but the tank is far more forgiving. Tropica's three tiers show where each plant group sits:

Even 3–5 mg/L of CO₂ helps a little, so a small supply is not wasted on easy plants; it simply is not required.

Plant groupLight per litreCO₂Examples
Easy10–20 lm (0.25–0.5 W)Not neededAnubias, Java fern, Cryptocoryne, Bucephalandra, mosses
Medium20–40 lm (0.5–1 W)10–15 mg/LMany stem plants and swords
DemandingOver 40 lm (over 1 W)15–30 mg/LDense carpets such as HC 'Cuba' and Glossostigma, red stem plants

How much light does a planted aquarium need?

Each plant group needs a different amount of light: about 10–20 lumen per litre for easy plants, 20–40 for medium ones and over 40 for demanding carpets and red stems. Choose the plants for the light you have or plan to buy, not the other way round. LED and T5 are the efficient choices, and a colour temperature of about 5,500–6,500 K suits plants and looks natural.

Light fades fast with depth and toward the corners. Moving a plant 10 cm closer to the lamp gives it roughly 40 % more light, so in a deep tank the carpet at the bottom gets far less than the numbers on the lamp suggest. These figures are rough guides: the old rule of 2–4 W per US gallon (about 0.5–1 W/L) dates from fluorescent tubes, and PAR measured at the substrate is a better yardstick, but none of our sources gave PAR tiers, so we do not quote any.

How many hours a day should aquarium lights be on?

Start a new tank at about 6 hours a day for the first 2–3 weeks, then raise it to 8–10 hours once plants are clearly growing. Sources differ: Tropica recommends this 6-then-8–10 plan, CO2Art suggests 6–8 hours, and ADA's own start-up runs 10 hours from day one. The short start is the safer choice, because new plants are not yet growing and algae would get the light instead. Put the lights on a timer so every day is the same length.

How much CO₂ do aquarium plants need?

Aim for 10–15 mg/L for medium plants and 15–30 mg/L for demanding ones; easy plants need none. Tropica's upper figure is 30 mg/L. Hobbyists often quote 30 mg/L as a universal target, but we could not confirm that from a source, so treat 30 as a ceiling to approach carefully rather than a starting point.

You cannot read CO₂ directly with a hobby kit, so measure it through KH and pH. At a known KH, the pH tells you how much CO₂ is dissolved; Tropica, for example, gives pH 7.5–7.8 at 12 °dKH as a medium-plant target. Measure KH before you set any CO₂ target. A drop checker filled with 4 °dKH reference solution does the maths for you by colour:

Too much CO₂ suffocates fish and shrimp. Raise it gradually, and if livestock gasp at the surface, turn it down at once.

How do you run pressurised CO₂ safely and steadily?

Keep it steady and ahead of the lights. Stable CO₂ matters as much as the level: a supply that swings up and down is linked to algae such as black beard algae. A typical setup has a regulator, a solenoid on a timer, a bubble counter and a diffuser or reactor. Good water flow carries CO₂ and nutrients through the plants; dead spots collect debris and favour algae.

  1. Measure KH and set a target from the tier your plants need.
  2. Set the solenoid to switch on about 1–2 hours before the lights and off about 1 hour before lights-out.
  3. Start at a low bubble rate and raise it gradually over days, watching the drop checker and the fish.
  4. Once it is right, leave the bubble rate alone and check it daily.

How much fertiliser should you dose in a planted tank?

Less than the label at first, and only as much as the plants show they use. Plants need macronutrients (nitrogen, phosphorus and potassium; Tropica also groups iron and manganese here, though botanists count those as micronutrients) and micronutrients such as copper, molybdenum, zinc and boron. Liquid fertiliser feeds through the water; root tabs or capsules feed through the substrate, which suits root feeders on inert gravel. A nutrient soil covers the roots for a while by itself.

In a new tank, give little or no fertiliser for the first 3–4 weeks while plants root; nursery plants arrive carrying nutrients. Then begin below the label dose and increase only when growth asks for it. Nutrients the plants cannot take up go straight to algae. Also watch the fish food: Tropica names overfeeding as the most common cause of algae, because the extra waste turns into fertiliser.

How do you start a new planted tank without an algae bloom?

Start slow on light and fertiliser, fast on water changes and grazers.

  1. Plant densely, including fast growers such as Egeria or Limnophila that soak up surplus nutrients.
  2. Run about 6 hours of light and little or no fertiliser for the first weeks.
  3. In a high-tech tank, run CO₂ from day one and raise it gradually.
  4. Change 25–50 % of the water several times a week for the first 3–4 weeks.
  5. Add Amano shrimp early, about 1 per 5 litres during start-up, dropping to about 1 per 15 litres once the tank is balanced; Tropica's trial found markedly less algae with more shrimp.
  6. When plants are growing, raise the photoperiod to 8–10 hours and start fertilising below the label dose.

How can you tell what your planted tank is short of?

Read the plants and the algae. Pale or see-through leaves point to a nutrient shortage. New algae usually mean the light has run ahead of CO₂ or nutrients: either lower the intensity or photoperiod, or raise CO₂ and fertiliser to match. Change only one thing at a time so you can see what worked.

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출처

  1. Light — Tropica Aquarium Plants
  2. Fertiliser and CO2 — Tropica Aquarium Plants
  3. Make your aquarium a success — Tropica Aquarium Plants
  4. Care — Tropica Aquarium Plants
  5. Algae control — Tropica Aquarium Plants
  6. Starting a new aquarium — Tropica Aquarium Plants
  7. Brief Guide on CO2 Injection in Planted Aquarium — CO2Art
  8. Comprehensive Guide to Managing Black Beard Algae in Aquariums — CO2Art
  9. Nature Aquarium Starting from Zero — ADA (Aqua Design Amano)
  10. Aquascaping — Wikipedia
  11. Liebig's law of the minimum — Wikipedia

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