I know this is an old thread but as I am a new member, and a sushi chef by profession, I would love to clear up some misconceptions about farmed salmon.
Like all things...there is good and bad. It is always good to trust your fish purveyor and the quality of what he sells. FARMED SALMON is the best choice for sushi chefs without professional experience. Most good fish farms today will guaranty their fish to be parasite and bacteria free). It is the only Salmon I serve in my restaurant, and everyone raves about it.
Salmon, especially wild salmon, can have parasites and worms from fresh water. If the farm salmon comes from a clean farmed source, it is wayyyy less likely to contain these parasites. Also, farmed salmon is more likely to be fattier, which means a richer and milder taste...Less fishy. There is a ton of misinformation out there. If you look for bargain fish...it is what you will get...One should never look to eat cheap sushi or sashimi... of any kind.
If your fish purveyor cannot guarantee that your salmon is bacteria and parasite free, then you should lightly cure the fish for a day under refrigeration (many explanations on line for this process) then rinse, dry thoroughly and wrap tightly in freezer plastic and deep freeze for at least a week in small hand size portions.
Remember...Once your fish is out of water...you never want it go back in again... rinse briefly, dry completely, wrap tightly.