For your program: do fix your indentation (ctrl-T in the IDE) and post with code tags.
For the hysteresis: the tank fill level IS your hysteresis.
if (level < MINIMUM) pumpOn();
if (level > MAXIMUM) pumpOff();
No risk of fast on/off switching this way - assuming a healthy difference between the two levels. To be really sure you may consider adding a delay between sensing a low level and switching on the pump, so it won't switch on due to a single stray reading.