I've heard this can be done is it true and what are the caveats?
The FDA has required all fish (with the exception of tuna) destined to be served raw in the U.S. to be frozen at a minimum of minus four degrees Fahrenheit for seven days or minus thirty-one degrees Fahrenheit for fifteen hours. Either process will kill any and all parasites inside of a fish.
Home freezers cannot freeze this quickly and so not only do parasites survive the process but the long time lapse of freezing creates long ice crystals that pierce cell wall after cell wall as they grow.
http://www.eatsushi.com/article.asp?X=628
I would not eat raw fresh water fish, if I had other options.Can it be done? Yes. Should it be done? No. Why would it be done???