I don't fish with live bait, but you might try to add some ice to the water to keep it cool.  Warm water does not have as much oxygen as cool water.  There is also an additive you can get to keep fish healthy in the live well.  I don't know what it is called, but you could ask at a sporting goods shop.

Hope this helps.

Gary

Adding ice and the water additive is key. You can buy the additive at Gander Mtn, Cabelas or most other outdoor shops. Another thing is to keep the minnows out of direct sunlight (covering the bucket with a white towel can help). 3 yrs ago I bought the frabil  Minn-o-life cooler with the airator. At $39.95 i thought it was pricey but now i believe it was well worth it. I have put 4 dozen minnows in it and kept them going all weekend with good batteries in the airator, adding some additive and a little ice

Another consideration is where you are buying the minnows. I usually try to get mine from a particular store but when i am pressed for time i go to a bait shop closer to home, problem is their minnows don't last more than a few hours. So, you might want to consider buying them someplace else to see if you have better luck.

Anyway, hope this helps.

Cabelas brand is called  the Oxygenator U2 Pro formula livewell treatment.

Frabil also offers a pruduct called the Aqualung packet, which are tablets that will increase the oxygen levels in the water as well as help condition the water by removing chlorine and other contaminates.  I have used both with good results.

GOOD ANSWER ONEMORE! When you go to the bait shop look down in that tank of minnows, when that person scoops the top minnows up, dont take them, the (HARD MINNOW) is allways on the bottom of his tank, and yes, the temp down there is colder, and they are the fresh ones they just got in that day, the older ones come to the top. I dont buy those tablets, i have a cheap BUBLEBOX, 6$ from cabelas? It is still gong for six years now? I freez a plastic milk jug and put it in my well , TEMP, AND OXIGEN, keeps them all day long, OLDGUARDANGLER is right on the money with city water.it has chloline, it takes oxegen out of your water, and they will die soon, if you  have city water, fill your bucket up slow with a SPRAY ACTION (it helps) but put that ice on the bottem,  jim