Plant deficiency guide
CO2 (Carbon) Deficiency
Also called low CO2, carbon limitation, unstable CO2
Stunted, small new growth and creeping algae despite good fertiliser, the most common cause of a struggling high-light tank.
TL;DR, CO2 (Carbon) Deficiency
This is not a mineral shortage, but low or unstable CO2 limits plants more often than any nutrient, so it belongs here. Plants slow down, new growth comes in small and deformed, demanding species stall, and algae such as green spot, green dust and black beard move in because the plants are too weak to compete. It is usually not enough CO2, CO2 that swings during the day, or poor flow leaving dead spots. Raise and stabilise CO2, get flow moving it everywhere, and use a drop checker to confirm a good level by lights-on.
What you see
New growth comes in small, pale or deformed, overall growth crawls, and the demanding species stall or melt while easy plants like anubias and java fern hang on. Algae builds on leaves and glass, often green spot, green dust or black beard, because weak plants stop out-competing it. It slows and shrinks the whole tank rather than targeting one colour or leaf age.
What causes it
- Not enough CO2 injected for the light and plant load
- CO2 not turned on early enough before the lights
- Poor flow leaving some plants in dead spots with no CO2
- Light too strong for the CO2 being supplied
- Surface agitation off-gassing the CO2 as fast as it goes in
How to fix it
- Raise injection gradually, watching the fish for signs of stress.
- Turn CO2 on one to two hours before the lights so it is at level by lights-on.
- Use a drop checker and aim for lime green when the lights come on.
- Improve flow so CO2 reaches every corner of the tank.
- If you cannot add more CO2, lower the light to match what you can supply.
- Treat liquid carbon as a mild supplement only, not a replacement for gas.
Keep it from coming back
Run CO2 on a timer that leads the lights, keep the drop checker lime green through the photoperiod, and match your light level to the CO2 you can actually deliver.
Easy to confuse with
Low CO2 and calcium both stunt new growth. If GH is fine and algae is spreading, suspect CO2. Nutrient deficiencies usually target a colour or a leaf age, while CO2 slows and shrinks everything at once.
Plants that show it first
Compare against a species you keep. These plants flag carbon trouble early or clearly:
- Dwarf Baby Tears (HC Cuba) — A carpet that needs steady CO2, it stunts and melts first when carbon is low.
- Monte Carlo — Grows leggy and pale, then stalls, when CO2 is short.
- Dwarf Hairgrass — Thin and slow to carpet without enough CO2.
- Rotala Macrandra — A demanding red that stunts and deforms first when CO2 swings.
- Alternanthera Reineckii 'Mini' — Small, crumpled new growth signals unstable CO2.
- Eriocaulon Cinereum — A CO2-hungry species that shows carbon problems early.
Frequently asked questions
Direct answers to the questions search engines and AI assistants surface most often about this species.
Can low CO2 cause algae?
Yes. When plants are carbon-starved they stop growing and stop out-competing algae, so green spot, green dust and black beard move in. Stable CO2 is one of the strongest algae controls there is.
Is liquid carbon a substitute for pressurised CO2?
No. It is a mild supplement and a spot algae treatment at best. Demanding plants need injected gas.
What colour should my drop checker be?
Lime green by the time the lights come on. Blue means too little CO2, yellow means too much and a risk to the fish.
Is it a nutrient problem or CO2?
Nutrient shortages usually target a colour or a leaf age. CO2 slows and shrinks the whole tank and brings algae with it.