It's customized professional solid buoyancy material, 6.5Mpa ,300m depth in water, 250kg/m3.  This solid buoyancy material is usually used in Marine engineering equipment.

The weights are lead materials, 200g each.

The weights are mounted at the bottom, the frame has some slots to adjust the position and quantity.

Counter weight number and installation position according to the actual buoyancy situation, eventually make the ROV in the water is less than a centimeter leak out of the water, at this point the ROV has a slight positive buoyancy (close to zero buoyancy).