I know there are several growers out there that have expressed concern over rising CO2 levels at night. Let me save a lot of time here and be very to the point. Originally i had CO2 levels rising to 1500 at night and returning to the daytime normal of 500 shortly after lights came on and the plants ( healthy in the middle of flower stage) started consuming the CO2. I have some old posts on this with more details. Now its gets interesting. Same room seedlings only a few inches tall, hydro grow, hydroton media, compressed CO2 tank, LED lighting, panda plastic on OSB walls painted white, concrete floor 1 year old. After transplanting them to their new home within a few days they took a nose dive. Upon further investigation the CO2 level was 3000 ppm. This obviously will kill a seedling. The CO2 tank in the room was already runed off as no additional CO2 is needed when plants are so young. In fact i completely removed the tank surmising it was leaking thereby causing CO2 level to rise. No affect. The door has been open a foot now for a week with a small floor fan bringing in outside air. The CO2 levels still hover around 450 to 550 ppm. I checked the CO2 sensor in another area and it works fine. Something in that room is emitting CO2, continuously. The OSB walls, painted with Valspar latex paint a year ago, the panda plastic, the hydroton stones, the plastic containers over a year old now, the bare concrete floor a year old, the fiberglass insulation? I am at a loss as how this much CO2 could still be in that room. I drained half the system (about 50 gallons) and replaced it with fresh RO water as the existing water i assume was ladden with CO2 as I had been unknowingly aireating it with 3000 ppm CO2 air, poison. I have ordered some CO2 absorbent crystals that i will hookup to filter the air being bubbled into the water. Maybe this will help. But, i really need to locate the source of the CO2. Any ideas? Anyone?