Holy snotrags, Batman! I just realised the ummm.. IMPLICATIONS of this:
yashiharu:
Now the pump works! (diagram attached)(various tests...)
Result: can't control the speed thu pwm coding
The SPEED? This is the first time you mentioned speed. [sigh] No, a relay can't do PWM. It can turn on and off but not fast enough to 'simulate a lower voltage.' Now I feel pretty silly what I wrote in that last post, not realising that IMPLICATION. 
You can use a relay to apply on/off power to something, but you need a further transistor (preferably a FET) to MODULATE the power, in PWM, so 'simulate' a lower voltage.
Check Reply #10...
dave-in-nj:
to run the pumps with speed control, you will be required motor shield, or create your own circuit.to turn pumps on and off with no speed control, you can use motor shield, or a relay or create your own circuit with a transistor or mosfet.
The motor shield includes an H-bridge, which include -bidirectional- speed control. A lone FET can do unidirectional speed control.