I have been into aquariums all my life. Got to your local pet store and buy a carton of freshwater fish salt. It's very cheap. Usual amount is 1 heaping tablespoon per 5 gallons. Treatments like 02 are helpful as well. You can get Stress Coat at the pet store which will do the same thing. They neutralize chlorine / chloromines and contain aloe to increase the slime coat. Use them together (Salt & Stress Coat) with a good aerator for the most success and liveliest minnows. Adding a bit of ice now and then to keep the water cool won't hurt either.

-Bill