The best way to keep them fresh is water replacement, rather than aeration. If your bait is in saltwater and you are fishing saltwater, that's fairly easy.
As for aerating without circulating outside water, it's a more difficult process. Aerating with airstones and pumps is really your only solution. Microbubbles are more a problem of esthetics.
As for 12v pumps, go to West Marine. Bigger boats need bigger pumps, and they sell them for very large boats (yachts). We have a spare one of these on the boat - to replace a washdown pump if/when it fails:
http://www.westmarine.com/webapp/wcs...classNum=14177
4 gallons per minute in a pressure pump.
Bill
215g FOWLR... and anemones, GSP, gorgonians... carp, that isn't FO!
"I killed my first SW Fish in 1971..."