I'm surprised no one has asked what the tank is made from :
this will determine any subsequent answer.
lets assume its a plastic tank with a small filling hole, so small that it prohibits reaching into the tank and holding a fixing nut on the inside.
so you need to fit a drain fitting from the outside. cut a perfect hole in the tank so a fitting will thread into the hole, screw it in and use a correct glue on the flange against the tank.
but this is toooo much hassle, so go with Shauns suggestion