Dear Jorge,
the API 650 Standard doesn't have any requirement on tank separation distances. Who use to have stringent requirements to be strictly followed are the insurance companies, and this for safety reasons.
Some engineering companies specialized in petroleum plants design, such as ABB Lummus, M.W. Kellogg, JGC Company, Technip, Snamprogetti and others use to have their own standards, which are considered confidential and are not disclosed to third parties. Frankly speaking, however, those standards are based on the insurance companies requirements.
Fortunately, insurance companies don't make a secret on the separation distance between tanks. Get in touch with a reputable one and ask them. I myself have designed a small tankyard based on information gathered from insurance companies.
Giovanni S. Crisi
Sao Paulo - Brazil