So today I woke up in the morning, turned the aquarium light on, and expect Cinder to come swimming to the front of his tank like always. When I can't find him after searching for about 10 seconds I notice a black mass behind my Nutrafin C02 bubble-holder near the top of it where the 2 black suction cups are. I immediately got a massive adrenaline rush when I realized it was Cinder trapped behind it!!!
I ran to get a knife to get the suction cups unstuck and hoped he would swim out and wasn't hurt. Luckily he did, and he had no injuries after I examined him for a few minutes. He did, however, take a big gulp of air as soon as he got out and stayed near the surface for a few minutes. I was soooo scared and I had no idea how long he was behind there, it could have been all night for all I know.
He must've swam underneath it where there's only one suction cup holding it on and swam to the top of it and didn't know how to get out...actually he might have known how to get out and I could be over reacting but there was only about a centimeter of space between the opening behind the holder and the substrate. Needless to say I pushed it down all the way so there's no way it can happen again.
Here's a pic of how the CO2 setup was before, I took it a week or two ago:
Make sure to push yours down if you have one too!