An 18 inch Varanus salvator is still very young if it has been kept properly.

Be wary of care sheets, most are bunk.

There is a website for SERIOUS varanid enthusiasts: varanus.nl
Sign up there and do a search on all of your concerns; caging, humidity, temperature, etc. Just be sure to use scientific names as it is an international site and proper nomenclature will ensure accuracy through different languages.

It is by far the best varanid site; good luck and congrats on your salvator; they are truly magnificent animals.

Chris