You had an ammonia spike
We started keeping tropical fish again after a thirty year hiatus and I've been painfully reminded about monitoring water quality. Now I'm applying those lessons to my bait tank.
The tip you gave was the** foul odor and milky color**. It doesn't take long for the ammonia to build up in an unfiltered tank with a lot of fish and once they start to die, it goes toxic in a matter of hours.
We also use well water, but it's extremely hard with a high pH, so I use a buffer to lower the pH a little. I have an inexpensive AquaTech fitler and a cheap aerator to keep the oxygen levels up. Even with that, we were losing minnows, so now I'm also adding beneficial bacteria (Cycle or ACT) to convert the ammonia from the fish waste into nitrites, then nitrates. In the aquarium, the live plants take up the nitrates. I honestly don't know how it's going to work in the bait tank, but fish can tolerate high nitrates better than ammonia or nitrites.. I also make sure I have the bait tank set up and running for a few days before buying our fatheads.
If you don't want to do all that, monitor your tank frequently to remove any dead fish. Keep your water cold and don't feed them to minimize waste. - Roberta
"Anglers are born honest,
but they get over it." - Ed Zern