Here’s another thing, and I don’t know if it has been specifically mentioned, but it has been alluded to as we all mentioned that him putting the girls in the tanks is an odd way to handle the situation (to put it mildly) if SW killed them:
If someone else killed his children, not only would he want the bodies preserved to demonstrate the crime scene and prove his innocence/show what happened, but also he would want a proper burial and grave for them, (or respectful cremation, etc) memorial service, etc, have them in a beautiful place buried with dignity, and if burial was chosen, have a place where he could go visit the grave, place flowers, have everyone pay their respects, etc. For my loved ones that have died, this was a VERY important part of the process for me.