My Geo sp. Red head Tapajos have that same soft spot, although not a whole as your has. I don't know much about HITH, but it could be just that. Your PH is kinda high for Geo's too. I would check your hardness. When my water was reall hard, the soft spot seemed "softer". If that makes any sense. I used a softener pillow and it seemed to help a lot. I also had flagstone in my tank that seemed to be leaching something into my water that made it really hard. I pulled that out and it dropped more. Basically, in my ramblings, I say lower your ph and check your hardness. Both GH and KH. Do some water changes too. Good luck.