From a fellow grower: What should the CO2 concentration be inside a grow tent?
Atmospheric CO2 is adequate; additional CO2 only helps when you are providing a huge quantity of light.
As long as the tent has proper air exchange, the plants have the CO2 they need.
They should join this forum. If they actually have enough light and a sealed space, we can talk about ppms of CO2. And if they don’t need to add CO2, we can set their mind to rest about it and help optimize the other areas of their plan.
Ambient co2 is generally around 400ppm. This is good for a ppfd average up to around 800 umols per second.