I just found your other thread with more info. Now don't take my word on it because I don't use co2 or a sealed room, so take what I say with a pinch of salt.
Could be too much, there are a few threads about which say 1500ppm is a waste or too much, those measurements are usually used in full flower. It seems its common to turn it off during lights out too. I have no idea how to go about this.
My best guess would be its too high ppm for such small plants, this is based on looking at other threads, I cant link them because they are from other sites. I came across one thread which a guy was trying to prove that 1500 was wasteful and that above that it can damage the plants, he was saying above 2000 it can kill them. The one picture I found on that thread showed a plant in a further state of decay than yours but it did have similarities.
The thread I came across was called " don't use 1500ppm"
There is another thread which contradicts this by Dr Who, now that maybe the same doctor who that is on here so maybe worth shouting him out. If it is he seems to have written a great deal on co2 and would be a good source of info.
Like I say, not a user of co2 so don't take advice off me but use it to do more research. Goggle co2 lights off and best ppm for small plants.
@Dr. Who Is that you who wrote the article on co2? You may tell me to keep my mouth shut on things I don't have experience with, that's fair enough, but if it helps the OP solve the problem...
good luck finding the source of your problem ligrow
Edit, since they use oxygen in the night period maybe that is causing or contributing to the problem. sorry I cant give a definite answer. At least it gives you a few things to research