When I do my water changes I like to add a little warm water with the cold to get close to tank temps.My hot water has some calcium in it from the hot water heater. Will this hurt the fish? I use Prime to treat the water for chlorine and chlorimine.
Really, if you have calcium in your hot water, it is in the cold also, you may just be more aware of it in the hot, because it builds up in the hot water tank. I can almost bet if you had it tested they would be the same. The only time calcium is a problem that I know of is if you are trying to raise discus or another fish that is super sinsetive to hard water, in which case if you invest in an RO filter system that will take it out of the water you fill your tank with.