two main things to consider, shape and flow.
dont have a tank anymore, dont have a boat anymore for that matter, but oval or round tanks offer no flat surfaces for bait to run into and injure themselves.... flow means you have to have enough water changes on your well per hour for your baits to breathe. the more bait you want to carry the more volume of water you'll need.
what's probably hapening to you is that your baits is freaking out swimming in all directions head butting the walls of the tank and in the process suffocating themselves.

try carrying less bait in your tank ( I know its easier said than done but a few lively ones are way better than a bunch of dead ones) and see how long they last then adjust the flow of your pump (s) to increase quantity.

also think about talking to one of the head honchos in the fish department of a pet shop (just dont tell him of your intentions with the fish) he should be able to explain to you the specifics of biomass / water volume / diluted oxigen ratios

I Fish...Therefore I Am