It is possible for a few species . Mollies mostly, a few other types . These "freshwater" fish are not actually fresh water these fish are brackish water fish and sadly the pet stores lie and say they are fresh water . So to answer your question can a freshwater fish acclimate the short answer is No . But you can acclimate a brackish water fish to a full marine environment. Over a period of a few weeks . I have done this with a group of bumble bee gobys, It is very possible