I will have to try that rubbermaid idea though seems
A old cooler also works and is probably better. I’ve used both. I leave the cover on, but leave it cracked so fresh air can get in and exchange with the CO2. I considered modifying my cooler by drilling a hole for the air line, but realized that would prevent the CO2 Oxygen exchange. The agitation and circulation of the water is really what keeps the oxygen levels up. Plus, I don’t want fish jumping out or critters getting in and drowning.
A pump from a pet store with a stone or a Frabill pump from the bait store will do it.
I have had 3 shiners and a dozen minnows survive for 2 weeks in the shade of the Florida heat. Think I lost just 1 shiner and 2 minnows. Granted, these are not the more delicate shiners you find up there. But bullheads are far more hardy.
You have to be careful with disease too. In warm water it’ll knock off the bullheads pretty quick because they are in close quarters, it spreads quick. This could be what is happening to yours.