I have a 650 litre ibc,
i simply put it down as far as my arm would reach,
yes , you need the locking nut as well as sealant, underwater sealant from b and q around 7 quid,
it heats all water above the element, mine is on a timer from 4am until 8am,
then i turn it off mix it up with a stick, or turn the pump on and re-send it back into the tank .... then turn on the element again for an hour.
but to be honest by the time you get to you first job, the hot water will mix with cold and give you nice warm water all day,
and i have lagged my tank.