If CO2 is out of the equation, it could be bacterial. I've kept fish in a bucket for a couple weeks with no airstone or surface movement, so I highly doubt it's due to filtration. Something is causing your O2 to drop to dangerous levels, and the only thing I can think of is bacteria.
Something you could try is draining the tank as low as you possibly can, let things dry as much as you can without letting plants dry too much, and then fill it back up. That will help remove a lot of possible issues.