Hi, birdartist. I've been able to keep my minnows alive best by keeping them cool. I mean adding ice to the water. Heat seems to be their worst enemy. I bought a little aerator from Walmart for about 10 bucks. It's run by a D size battery with room for a spare. It works OK. I don't let it run constantly, but enough during the day and even overnight to do the job. The real key though is keeping the water cool.
Back in early April I went fishing on a Tuesday and used most of the minnows. There were two left when I went in. I left them in the bucket, gave them a shot of air for 30 or so minutes and then left them in the boat. The next morning the temperature here in Florence was about 58-60 if I remember right. The two minnows were still swimkming around the next morning.
That afternoon I again aerated them for half an hour and them closed them up. I forgot about them after that. The temps never did get over 65 during the day with the evenings dropping down to the low 50's.
The following Tuesday I was going out again and when I took the cover off the boat I looked in on my two minnows expecting them to be dead, but they were still alive and really active. The only thing I noticed was what looked like minnow crap floating throughout the water.
All I can figure is the cold. Any way that's my advice and I'm sticking to it!