A security lock for a storage tank includes a lock ring configured for engaging an opening that provides access to the storage tank. A suitable mounting assembly is used to mount the lock ring to the opening. One or more obstructions are disposed within an inside (and/or outside) area of the lock ring. The one or more obstructions define one or more openings that are sized to allow storage tank contents to be introduced through the security lock while blocking pipes, tube or other implements that are larger than the openings so that such implements cannot be used for siphoning or otherwise removing the storage tank contents.