Planted Tank Obsessed
Join Date: Oct 2005
Location: Bunker Hill, WV
Yes, they need dissolved O2 in the water to breathe. Water can be saturated with both CO2 and O2 at the same time, one does not displace the other.
In the daytime your plants are giving off O2 and adding it to the water. Sometimes so much that the water can't absorb any more of it and pearling is seen.
At night, the plants are using O2 instead of adding it. Thats the time you may see O2 starved fish, and need to run your airstone.