http://www.alldatasheet.com/datasheet-pdf/pdf/23381/STMICROELECTRONICS/MTP3055E.html

Here is a TMos Fet (T channel Metal Oxide Semiconductor Field Effect Transistor) Data sheet for the MTP3055E transistor.

It acts a lot like a relay, and will directly switch 12 amps of load at up to 60 volts, with a single microprocessor pin driving the gate. Your active led needs to be closer to the pump, to show actual operation of the pump. 

**This device has 3 leads:**
*Gate (controls  the switch like a relay coil))
*Source (Positive lead on device - like relay contact)
*Drain (Negative lead on device - like relay contact)

If you place the Source under the ground side **of the pump and solenoid**; and connect the Drain to ground, and lastly connect the Gate to your switch.  When you power the gate, the MosFet will turn on, and an active LED across the pump will light when it runs.

The relay can be scrapped *(this is MUCH more reliable);* but get the boost running before you worry about indicator lights. You will need to heat sink the MosFet, at 5 amp load it will disapate under 3 watts as heat. 

(P = I squared X R) where R = *Rds on (resistance drain to source when turned on) of 0.1 ohms.*