The common options are tap water, rain water, reverse osmosis water, and distilled water, their purity from lowest to highest. Do not use softened water because they are softened with table salt.
Fish tank water should be fine if you are not adding salt. Do not use the water if you're using one of those water softening pillows because they use table salt to replace the hard water salts! Fish don't react to NaCl the way they react to the hard water salts. NaCl equals disaster in orchids! (and not only for orchids, although some houseplants are more tolerant of NaCl)
I use tap water. I let the water sit in jugs so that it get to room temperature and the chlorine can disapate. I am hearing that some local growers are melting snow water for slipper orchids because the water is being treated differently, but I didn't get enough detail (something about chlorine. I would guess a switch from chlorine to chloramine? Having aquariums too, I read that chloramine doesn't disapate, but can be neutalized, which I barely do for my aquariums unless I change large portions of water. My plants/aquariums aren't dead if there was a chloramine switch). If I ever needed to change the water I used, I would buy reverse osmosis water and mix it with the tap water. I think its better to mix some tap water with the purified water because it buffers it. Melting snow would take long hours to get enough of it.