so long as you keep the tank clean and it's still swimming?
the salmon will be fine after one day. like breezy said, the only "fishes" that need to be consumed that same day are live things like crabs and lobsters. slaughtered, er, "cleaned" fish is good for a day in the fridge or on ice; two at the most.
unless you really do have a tank.
this reminds me of the quote from ben franklin: "houseguests, like fish, begin tlo stink after three days"...