..... and if the tank or tanks had fuel in it (them), then the calculations change yet again! Gasoline is lighter than water so it should help the tank float. Didnt a deep submersible (Trieste) use gasoline in a bladder above the capsule to provide floatation?
Yes,
the Trieste did using gasoline in its float:
"The sphere weighed 13 metric tons in air and eight metric tons in water (giving it an average specific gravity of 13/(13-8) = 2.6 times that of sea water). The float was necessary because of the sphere's density: it was not possible to design a sphere large enough to hold a person that would withstand the necessary pressures, yet also have metal walls thin enough for the sphere to be neutrally-buoyant. Gasoline (petrol) was chosen as the float fluid because it is less dense than water, yet relatively incompressible even at extreme pressure, thus retaining its buoyant properties and negating the need for thick, heavy walls for the float chamber. ...
"Nine tons of magnetic iron pellets were placed on the craft as ballast, both to speed the descent and allow ascent, since the extreme water pressures would not have permitted compressed air ballast-expulsion tanks to be utilized at great depths. This additional weight was held in place actively at the throats of two hopper-like ballast silos by electromagnets, so that in case of an electrical failure the bathyscaphe would rise automatically to the surface."