I can't see the subfloor being 1.5" thick...where are you able to see the subfloor Coleus? If it is indeed 1.5" thick you don't need to do anything to the tile. It won't deflect enough to dislodge the tiles or crack the grout.
As far as the actual weight of the tank, I told you from the get go that you don't need to support the floor. Thankfully some other people have come in here and supported this as well.
