Ralph,
My guess would be to run each instance of the aquarium in windowed mode & to be sure to open only one instance of each different version of the aquarium (ie. only one instance of SSAv.1.1, one instance of SSAv.1.1-Widescreen, one instance of MAT & one instance of SSAv.2.) This has worked for me & I am using Windows 98 SE. Hope this helps!