The bounding box on my sims render with an edge

Not sure about x-part stuff. if not needed start a new scene without xp volumes.
For the shading start at x,y 0,0 and increase the x to cut into the fluid more. All other render engines work on a similar concept the graph may be a black to white gradient. Or even a direct numerical input. You’ll see what i mean if you do a standard density sim then adjust the y value at the left

https://help.jawset.com/en/c4d/fcurve_editor