VDC for little centrifugal pump. How much tolerance?
You mustn't run a 7V rated device from a 9V power supply. It will probably work, but it will have a much reduced life time.

Cheapest solution that is somewhat reliable is to connect 3 diodes (1N4001 - 1N4007 will do fine) in series with the pump. These will always drop about 2V, in contrast to a resistor that drops voltage depending actual current. 







simulate this circuit &ndash; Schematic created using CircuitLab

With a resistor, when the pump runs dry, current will probably drop and voltage on the pump will increase and the pump may get damaged.

Best solution from efficiency point of view is to find a power supply that outputs 7V regulated or add a step down converter. Something like this:



which usually sells for couple dollars/Euro.