Question

source of CO2 for aquatic plant

Answer 1

Plants need carbon
to create their food
(photosynthesize). They
obtain carbon from
either carbon dioxide (CO2)
or some plants can take
it from carbonate hardness (KH).
It is easier for plants
to utilize carbon from CO2,
which is naturally present
in the aquarium, but not
usually at the levels needed.
As CO2 levels disappear,
plants slow their growth, forcing
them to use the carbon
from KH, which is the ingredient
that holds pH stable. When
this buffers content is lowered,
pH levels can change dramatically,
which may severely stress
or even kill fish.

Answer 2

aquatic plant take CO2 that remains in the water in dissolved condition,these CO2 are mostly exhelled by the aquatic animals.