It's a 6w pump...it should be able to run fine off any 3 or 4 pin motherboard header.
Power it up with an adapter to ensure that it functions. If it doesn't something is wrong with the pump or there is an obstruction.
If it does power up when run off just 12v, your PWM settings for your fan headers are probably too low for the pump to start correctly.
...rightful liberty is unobstructed action according to our will within limits drawn around us by the equal rights of others. I do not add 'within the limits of the law,' because law is often but the tyrant's will, and always so when it violates the right of an individual. -- Thomas Jefferson