Normal *cold* tap water should have much lower levels of copper/metals in it than water that has been through the water heater. Warm will have less than hot as it's less of a concentration of water heater treated water. I would just run the tap on cold, fill up the buckets and let them age, they'll come up to temp pretty quickly. Also I don't even allow the water to warm up when I add it to my 10 or 20 gallon. It's dechlor in the bottom of the jug, run the tap on full cold, swirl it on the way to the tank, and dump it in. I noticed my bettas seem MORE active since I started doing this, I don't know if the temp swings more naturally mimic the weather cycles or if I was getting contamination in my hot water.