Hi to anyone reading, Don't know if the topic got a resolution so here is a thought for you. Either get the Leviatan carrier mod or create a resorces config file that has this.
If you don't already know you just open a new text document with Notepad [assuming you are on a windows pc] copy and paste this and save it with a cfg suffix. eg Resource.cfg.
The next can be done with any part but for me I chose the Baguette radial tank which is stock and found in Gamedata/Squad/Parts/FuelTank/FoilTanks/RadialTank_Capsule.cfg . Open it and then immediately "Save AS" something else. In my case I called it, RadialBallast_Capsule.cfg
Always do that so you don't bugger the original. Then amend or copy and paste the following to create your new part.
If you copy and paste then make sure you replace all the existing text so it only looks like the above. Save it and leave it where you found the cfg file so it uses the right mesh and textures. If you have tweakscale it will be resizable. It can then be found in game by typing ballast in the search. Attach to the bottom of your sub on the keel [centre bottom] then use the move tool to move it just far enough up to not be visible. I find it best to add liquid after launch. Using the fill or empty commands. They are separate functions so don't turn them on at the same time or the level won't change. So radio buttons if you right click on the part or pre assign to a custom function when creating in the hangar. I find 1 of the correct volume works fine or two , 1 front and 1 back and remembering to turn off flow to 1 tank at a time with the green triangle, is good for leveling. Mechjeb2 can give you a vertical velocity dislplay that will allow you to get within mm/s of neutral buoyancy. Enjoy.