Not cannot open.. is closed too tight.. happened to me once . You don't have to close it too tight.. just finger tight
There's 2 ways now.. either u use 2 pliers, one holding each side and open.. remember to use a cloth in between to protect the tank.. otherwise u may try using a metal rod .. insert into the air hole.. and try to turn it out.. the metal rod act as a lever .
I do not know if soaking in hOT water helps to expand anot.. u may try also.