This would really be governed by how your tank is actually supported. Uniform soil support underneath the bottom slab of the tank is generally modeled using PLATE MAT. All you need to do is specify the soil subgrade modulus and the software would calculate the spring stiffness at each node depending on the plate area that is supported by the node. There is nothing special you need to do for corner nodes as these nodes will automatically be assigned the springs of appropriate stiffness depending on their respective area of influence. The Section 5.27.3 of the technical reference manual has more details on this type of support. In addition you would generally specify the COMPRESSION only attribute for these springs as soil would not resist any uplift.
If you have anchors or piles at certain locations supporting the tank then you can model these as spring supports ( FIXED BUT type supports ) and assign the appropriate stiffness to these springs. Depending on the type of support, you may have to assign it a TENSION or COMPESSION only spring too. For example if it is an anchor that would just come into play when the base is subjected to uplift then you would specify the support at the node as a TENSION spring. More discussion on compression or tension springs can be found at section 5.27.5 of the technical reference manual.