Keeping minnows alive in pond

I have been trying to keep my extra minnows I have after my fishing trips alive in the pond at my house. I got some 6 days ago and have had probably 1/8 of them have died. Water temp is probably 65-70 degrees. Do you think they are dying because it is to warm? Pond is aerated but is dirty from all the rain we have had in Iowa. I just hate wasting minnows.

I am wondering if I would be better off getting a cheap aquarium and keeping them in the basement where its cooler.

A guide at crappie university said to take a regular cooler and install a aerator in cool place and you should keep them alive no problem.

PH maybe.... Ammonia… overfeeding

I have 2 kinds of minnows (not sure what species) in a turtle pond and they do fine in the winter and summer. Out here in SoCal it gets HOT. The pond gets lots of aeration.