If you're looking to deactivate one of your Oil or Water tanks, you can do that from the Wells module on your iWell account.
1. Go to Manage, select Wells
2. From your list of Wells, select the one you want to edit
3. In the Tanks/Meters and Allocations tab, select the tank you want to deactivate
Once you've deactivated the tank, it will be removed from every user account in iWell - don't worry, you will not lose previously saved data.