Get a floating thermometer so you know what the surface temp is.
Marina Aquarium Floating Thermometer w/ Suction Cup Suction Cup Thermometer - Walmart.com
Ok , after seeing the minnow tanks made from blue barrels I had to make one myself.( thanks gilman and others) But after I got it built and running I decided to try to insulate it. I probably just wasted some time since I have no idea whether this will help or not but I wanted to try. In case anyone is interested I used the pink fanfold 1/4 inch thick foam board insulation that they use under siding to start out with. I cut the heighth down to 32" and used 50 feet of length which is the length it comes in per package. I think this got me about seven revolutions around the barrel. After I had it wrapped tight and secured with duct tape I covered it with one layer of reflectix bubble insulation. I used foil tape on those seams. At the top I sealed up the exposed edges of all those layers of insulation with duct tape and sealed against sides of barrel.
I've had mine running about three weeks now with three dozen minnows left from a fishing trip and it's been working well. I have two 10/30 gal. aquarium filters from Walmart with an air pump with a 10"stone .
I realize this probably isn't gonna help a lot when it gets hot and stays hot but I thought it might help a little.
Get a floating thermometer so you know what the surface temp is.
Marina Aquarium Floating Thermometer w/ Suction Cup Suction Cup Thermometer - Walmart.com
Hotpepper LIKED above post
Get a digital aquarium thermometer and leave it suspended half way down in the water. They are about $5.
Isn't your reflectix on inside out? Isn't the shiny part supposed to reflect the rays and heat away from the bin? The dark brown will ABSORB heat. You want to keep the minnows cool, 50-60 degrees (colder the better)
Sorry guess picture not the best but the reflectix I used is actually shiny foil on both sides. The white strings at the barrel rim are holding two pool thermometers. One about 6" below the surface and the other about 6" off the bottom. So far they have read the same almost every time except a couple times there has been one degree difference. It's been cool here so water has been 49-50*. I'm just experimenting here but so far the tank is working great.
Have a thermometer hanging close by to measure the outside and compare to tank temp. The best I could do was 10 degrees cooler when it got hot.
Hotpepper LIKED above post
I like your setup. I may have to try to do one later.
FISH ON
Minnows will easily live at 70 degrees.
KJV Numbers 32:23 " be sure your sin will find you out."