1/3 water rest air.. doesn't have to be O2... and have 'em double bag it... a leak mid-drive = pain in the behind! I've routinely bought fish 1 hour or so drive away w/out any major problems. be careful of squishes... bring a box or cooler to keep the bags secure and so they aren't rolling around on the drive ect.. take some common sense precautions and you shouldn't have an issue... not sure if 02 is such a great idea for air breathing fish such as polys or not. drip acclimateing new fish.. 'specially if they are straight from the distributor.. or in a water area where the water chemistry is different (different sewer ect)