i use a rule 700 gph thru hull bilge pump attached on the outside of a plastic 55-gal drum...i like to keep the pump outside of the tank to keep from heating up the water and to provide easy access to the pump for maintenance...the pump inlet tube runs through the side of the drum and connects to a short (6") length of perforated hose that extends along the bottom end of the tank...this length of hose prevents a dead fish from completely clogging up the inlet during travel (don't ask how i know
)... the pump outlet tube enters near the top of the tank and sprays water back into the tank, providing aeration...
i've kept 12-15 herring in a 20gal tank for 12 hours but thats maxed out...30's good, 50's better (and then some guys have portable tanker trucks
)
keep the water cool and clean and you'll increase your survival rate...i don't do overnight storage in my tank anymore but even with the 55 gal drum i probable had 25% mortality overnight...