Can't Figure what I did

Hey ... I'm a so-so excel user and I created this one file a while back.

I'm trying to figure out what I did to get a drop down combo box.

I know how to add one through the forms toolbar, but I can't figure how to copy an old one and reconfigure it to work with a new list.

Under "Fresh Tank Populations" sheet, Column C is loaded with drop-down combo boxes that pull from a list on the "List" sheet.

I'm trying to have the same thing work under Column C of "Salt Tank Populations" ... but I can't figure out how to change the reference table on the "List" sheet.

Any help would be much appreciated.

Thank you.