I use heated water for my tanks. I was concerned about the hot water sitting in the tank, and containing traces of copper, so I bought a test. It was negative for any traces of copper. If you are concerned, buy a test kit and find out for yourself.
As for letting water sit out for 24 hours, this mostly has to do with "gassing off" any residual CO2 that could lower your pH, and in order to stabilize the pH for aquarium use, aging the water with a bubbler is what a lot of people do.
Aging your water without Prime is fine, as long as your municipality doesn't use chloramine. However, most municipalities do use chloramine, and an agent like Prime is absolutely necessary.
When doing water changes, I siphon the old water, then add Prime for the total amount of water in the tank (old water and new water) then adjust the temperature from the sink, and fill the tank.
To OP,
I highly recommend checking your nitrates in your tanks, 20% is probably not enough even if done weekly, nitrates climb, and the best way to keep them down is with frequent water changes. If you work out the math, you will see just how insufficient 20% weekly really is and you will want to lower your nitrates before your fish become symptomatic of stress/infections.