I think it will not be in any code, the NFPA 30,20,22 give guides for common buildings but they will not include an oil tank in a code line. They say it "shall not be exposed to fire", and an expert or AHJ shall define what is "expposed to fire" in your particular tank installation. I´not 100% sure, but I think nobody will define a general distance for locating a fire pump from a product tank, this distance will always be related to specific conditions or analysis.
API 2001 (API RC for refineries) does not tell a number it says that the layout of the plant has to analyse all the risks to be safe water source shall be as far from risks as possible.
I think that if you use NFPA 30/NFPA 20 distances it will be a risky minimum but I would include, revision of this distances considering dikes, walls, oil product pipes, etc. If your water tank/pump is donwind or downward natural drain direction in case of a spill the NFPA 30/20 distance will not work.
From NFPA 20 (2004)
5.12.1.2 Outdoor Fire Pump Units.
5.12.1.2.1 Fire pump units located outdoors shall be located at least 15.3 m (50 ft) away from any exposing building ( i´m sure a product tank cannot be interpreted as an exposing building).
5.12.1.2.2 Outdoor installations also shall be required to be provided with protection against possible interruption in accordance with 5.12.1.
5.12.1* General Requirements. The fire pump, driver, and controller shall be protected against possible interruption of service through damage caused by explosion, fire, flood, earthquake, rodents, insects, windstorm, freezing, vandalism, and other adverse conditions.
NFPA 22
4.2 Location of Tanks.
4.2.1 The location of tanks shall be such that the tank and structure are not subject to fire exposure.
4.2.1.1 If lack of yard room makes this impracticable, the exposed steel work shall be suitably fireproofed or shall be protected by open sprinklers (see A.12.1.1).
4.2.1.2 Fireproofing, where necessary, shall be provided for steelwork within 6.1 m (20 ft) of exposures, combustible buildings or windows, and doors from which fire might issue.