You could use a down flow system but I would recommend a fleck 5600 flat cap, (filter valve) It will also be okay to backwash onto the ground outside. Also if your hardness is 4gpg before you added calcite it is probably around 7+ now. Your tank size is okay but perhaps you may want to take out some of the calcite first before you start adding or changing anything. I would measure your ph and see what it is as well as your hardness. If ph is too high then take out some calcite, you may only need one cube and it sounds as if you have almost 1.5 cu ft of calcite.