Hi,
Additional to FTank's comments above, if you use a cone down foundation you can install tell-tale leak detection pipes in the foundation (in accordance with Appendix I, and as long as the pipes are kept free from debris and dirt any water underneath the tank should drain via the leak detection system.
Further to that I am not aware of any API standard or RP requirements but generally we ensure that the tank foundation is sufficiently above the compound floor (i.e. 0.3 - 0.5m above the main compound).
The other thing is to seal the floor plate projection (chine plate) to the foundation. There are a number of products on the market for this i.e. Shell Tixophalt, SikaTank, etc. We generally use Tixophalt, and paint it with an aluminium reflective paint to prevent it from prematurely failing due to UV.
There are also other products such as the Denso "Tank Boot" systems. However we have found that these systems tend to hold dirt and moisture unless they are regularly cleaned.