Probably no, as you are more than likely going to bring in some nasty living "other thing", whether it be excessive algae spores and even worse some insect eggs/larvae that will thrive in your tank. You could chlorinate then dechlorinate or UV sterilise before use, but I think you will find tap water (and dechlorinator) very cost effective and safe.Is such water useable in an aquarium?
I can see Ian's reservations, but I wouldn't be too worried about them if the source of the water is a trout stream, because as you imply the presence of Trout is a pretty good indicator of water quality.The lakes I fish are gin clear and the one river is a classic chalk stream with a good flow. Is such water useable in an aquarium?