Mussels are bad when they are dead and when the mussel floats.
You should always discard any floating mussels that float to the top of the water as the floating mussels indicate the mussel is dead.
Good and alive mussels should close by themselves, and although some might close slowly, they are still good and alive.
If the mussels do not close, throw them out.
Discard all mussels that have broken shells or an unpleasant odor, as well as any that feel unusually heavy or light compared to the rest.
Mussels that are floating are dead and should be thrown out.
Give any open mussels a sharp tap with a knife, and discard any that fail to close (they are dead).
Place cleaned mussels in a fresh bowl of cold water until ready to use.
Change this water two or three times to remove any salt or sand that the mussels may expel.
When the mussels are alive they should not float and you should keep the mussels alive until you're ready to cook them.
Dead mussels could contain bacteria and parasites which can make you sick so you should throw out any dead mussels that are floating.
When the mussels float they are either dead or empty and if the mussels float they are bad and should be thrown out.
The mussels should not float and if the mussels do float it's an indicator that they are bad and dead and should not be eaten.
Eating already dead mussels even when cooked can make you sick because they can contain bacteria.
Partly opened mussels can also be dead and to test whether the mussel is dead or not squeeze the mussel shut and if it stays closed then it's good and if it does not stay shut then it's bad and should be thrown out.