Protecting arduino (destroyed two) when controlling pump with MOSFET
Your circuit looks notionally OK.
You are probably doing something other than you THINK you are.
Load turnoff inductive spike is fatal if not properly limited. D2 should work if properly located.
Be SURE D2 really is across the load.  

For playing and especially for just on/off as opposed to PWM switching drive gate with say 10k to isolate uC from gate.

Gate can get Millar capacitance coupling from turn off spike on drain. D1 limits this to Vin + Vf_diode or about 14V say at 12V supply. This rise form 0V MAY be enough to couple a low energy spike into drive cct. 

Nicish MOSFET  notice Vgs_abs_max is +10V - as well as drive resistor above, clamp gate with a reverse zener, gate to source to prevent Millar capacitance coupling inductive turnoff kick into gate AND into Arduino.
IF MOSFET is destroyed by spike it can short drain to gate and controller will die. Was MOSFET OK?

Using supplied schematic editor will allow a MUCH clerer diagram to easily be drawn. People will greatly appreciate the difference. You are welcome to copy and adapt my one if desired.

R2 is for protection only against short transients and will need to be "designed" for eg PWM operation.
R1 needed only to ensure no turn on at start up with uC asleep.
Zener D1 clamps gate to below Vgs_abs_max and also to limit spike feedback to uC. 

Components in dashed box at left add power supply spike and surge filtering. 







simulate this circuit &ndash; Schematic created using CircuitLab