That fish tank deduction sent chills down my spine (especially when I doubled back to reread the ch and saw the fish tank in all its glory) since it utilizes good, old fashioned logic.
I already thought the two who served sake to Kindaichi was suspicious enough since that kept him out of the bath for a good 5-6 hours, plenty of time to redo the garden and if the garden WAS redone, the two disciples lied about "nothing seemed repaired."
The remaining daughter would probably be super traumatized and the mother choosing to not be there for her feels pretty unloved. Sure, she cannot change the past, but she can still change the future. >.<
@cubia: At your comment, I reread the chapter to look for the hint and since the author referenced what Mizuki smelled in the earlier chapter again, is the sour smell the big hint??? Is it from a solvent that hid Ruri's true time of death? I found Izumi-san's comment about Onodera-san's looking...