What low, medium and high light and CO2 mean in grower plant data

The scale on every Tropica plant page

Tropica rates each plant’s light demand and CO2 need as low, medium or high, and its difficulty as easy, medium or advanced. The definitions are in the help text attached to every plant page ^[1]:

• Light demand: “The average or medium light demand of an aquarium plant is 0,5 W/L.”
• CO2: “A medium need in CO2 is 6–14 mg/L. A high demand in CO2 is approx. 15–25 mg/L.”
• Height: the average height of the plant after two months in the tank.

These ratings are what this site shows in the spec card on each plant profile and in the parameter table.

Three pages, three sets of CO2 numbers

Tropica publishes CO2 guidance in at least three places, and the figures do not line up:

Tropica page	Easy / low	Medium	Advanced / high
Plant page help text	not stated	6–14 mg/L	approx. 15–25 mg/L
Fertiliser and CO2 guide	no added CO2 needed; 3–5 mg/L “better than nothing”	about 10–15 mg/L	15–30 mg/L
CO2 System Nano page	approx. 3 mg/L	approx. 10 mg/L	approx. 25 mg/L

Sources: ^{[1, 2, 3]}.

The plant pages rate CO2 need (low, medium, high) while the guides speak of difficulty groups (Easy, Medium, Advanced). The two scales often coincide, but not always: Tropica rates Sagittaria subulata easy yet medium in CO2 need. Read as a whole, the three pages agree on direction (more CO2 for more demanding plants) and disagree on the lower bounds by a factor of up to about 1.7. Treat them as bands, not targets.

Aquasabi’s version

Aquasabi’s wiki describes the same traffic-light idea, which it says nurseries such as Dennerle and Tropica use, and attaches its own numbers ^[6]:

Category	Light	CO2
Green (easy)	about 0.2–0.3 W/L, or 15–25 lm/L for LEDs	around 20 mg/L recommended
Yellow (moderate)	about 0.4–0.6 W/L, or 30–45 lm/L	around 20–30 mg/L
Red (demanding)	from about 0.7 W/L, or 50 lm/L	around 30 mg/L

The light bands agree with Tropica’s 0.5 W/L for medium. The CO2 recommendations do not: Aquasabi suggests about 20 mg/L even for its easiest group, several times Tropica’s 3 mg/L for Easy plants. Aquasabi frames that as a recommendation for better growth rather than a minimum; it says most green-category plants can do without carbon fertilization but grow significantly better with it ^[6].

Watts per liter in an LED world

Both scales still express light in watts per liter, a rule of thumb Aquasabi traces to Paffrath in 1979 ^[7]. Watts measure power drawn, not light delivered, and LED efficiency varies: Aquasabi quotes roughly 120 to 180 lumens per watt across 2018 models ^[7]. The same 0.5 W/L can therefore mean quite different light at the plants. Aquasabi’s lumen-per-liter bands are one step better for LEDs; a PAR meter reading is better still (see PAR, PPFD and DLI).

Reading the ratings sensibly

• A low rating is a floor, not a promise of appearance. Tropica rates Rotala rotundifolia low in light demand, yet its description says it needs good light to turn red ^[5].
• The grower’s baseline is brighter than “low” suggests. Tropica’s FAQ says its plants generally thrive in moderate to high lighting ^[4].
• Rate the tank by its plants. Aquasabi suggests counting how many plants fall into each category and treating the largest group as the tank’s overall requirement ^[6].