|If you're unsure of the accuracy of your monitoring software, try checking the temp readings in the BIOS.|
"I'm only running the five fans that came with the tower"
Only?? I only run ONE case cooling fan....120mm exhaust in the rear. All my towers (4 systems) have top mounted power supplies so the PSU fan acts as an additional exhaust, no other fans are necessary. Intake fans generally aren't required however a direct source of fresh air to the CPU via a duct in the side panel is a good thing to have. No fan in the duct is required. Assuming the opening in your side panel is directly above the CPU, I would remove the fan & replace it with a duct.
"I'm thinking about replacing the five fans, and adding five others. Do you have any suggestions?"
Waste of time & money. You already have too many fans. And your CPU temp is fine, so I really don't see the need to make any changes other than the air duct.
"personally I idle at around 20° - 25°"
Since average room temp is 20°C (68°F), it's highly unlikely your CPU idles at 20°.