Switch the mode to "View". Click on the "Eye" button. Change the values for the "Clip Planes" so that they are away from the fish. The "Far" plane should be well behind all geometry and the "Near" plane should be well in front of all geometery. In other words, redice the "Far" value and increase the "Near" value. Then click on "Ok".
Regarding the STL file input/output -
I've never experienced any issues with it. The STL file format is simple. Perhaps the other programs are expecting an ASCII version instead of binary, or vice-versa. Another possibility is that there are too many triangles in the file for those programs to handle. When exporting STL files directly from VS3D, the total number of triangles depends upon the grid size. Resizing the grid to smaller dimensions ("Edit / Resize Grid..." menu) and then exporting STL will result in a smaller STL file with fewer triangles.