You probably want to open it every so often to let fresh air in. Not sure how much air is in there vs space taken up by him and the water. If you have some way to put something under him that leaves air space (like a little wire basket upside down or something to hold him towards the top of the container so that CO2 falls down below and he is still getting O2 that would help. In shipping I've had plenty of animals come in boxes with no air holes poked in, but they are only taking up about 1/10th of the space in there and the rest is crumpled paper (mostly air).