Although there is "true cod", cod is also a catch-all name for lots of
different fish species. Without knowing more about what sort
of fish this was supposed to be, it's hard to know what's going on;
however both true cod and anything called cod are usually your
stereotypical "flaky" fish, when cooked normally. Neither fresh
nor frozen cod should be tough. As mentioned upthread it can
get a little chewy if it's been smoked/cured. (But not if you
do it right; I've smoked black cod with excellent results.
Smoked black cod is called sable.)

Steve