My best results (for the last 12 years or so) are using floating plants like water sprite as these will form thick mats that the caecilians will then crawl up into and rest. (It also helps as it gives any babies a shallow refuge to rest close to the surface.
I would not recommend using CO2 as yes while CO2 is picked up during the day it is relased at night. In addition additionally CO2 is really only needed if you have an extremely high plant growth (like a Dutch set up) and want to keep it at a high rate of growth.
Ed