Depends on what your trying to do? How many plants and how big? Outside air is about 400ppm, that is fine to grow with. Typically we try and get to about 1500ppm when adding co2. But yeah, 400ppm is just fine. In a sealed room with no co2 injection or Outside air the plants will deplete the co2 pretty quick. When my burner is off my co2 gets to about 120ppm