I didn't really understand that either(English is not my native language).

Could you make a new drawing with those diodes included where they should be?

I didn't really understand that either(English is not my native language).

Could you make a new drawing with those diodes included where they should be?

When the contacts in the switch or relay open, the inductance in the pump and relay coil will cause high voltage spikes which cause sparks between the contacts. With enough on/off cycles these sparks will eventually erode or burn the contacts.

Snubber diodes short out the energy stored in the inductance when power is removed and eliminate the voltage spike. This is very important in circuits where a high voltage spike would damage the circuitry - in your circuit it is not essential but will reduce electrical noise and make the switching devices last longer.

Thanks again!

Which kind of diode should I use? could you recommend some?

I do understand enough to use it now, but I do wonder:

When you turn off the pump, it will "produce" som ind. current, which will go to the relay or back into the pump?

You paid for the electricity when you turned the pump on.  Have you ever plugged in a fan and seen a spark?  More likely when you unplugged it.  When you plug in an old stereo, the spark comes initially, 'cause it's a capacitive load.  There are two forms that analogize to mechanical inertia in electronics, magnetic and charge.

Vishay VS-6TQ035 might be a good choice for your pump.

1n914 would work to clamp the coil if it uses less than an ampere.

Some other alternatives for diodes

pump - 6A05-T from Diodes Inc

relay - 1N4002 from Micro Commercial, Diodes Inc or any other mfr

The inductive current spike will just recirculate through the diode and coil until it is dissipated as heat.