Ouch, shocked !!

[spreerider]

you can get voltage in the water without a actual leak, the AC current in the motors can induce a voltage in metal equipment that is within the electric field surrounding the motor. i would check to see if the pumps are close to a motor or if your ballast is close to metal in the tank.
the reason an arc fault trips on a MH is that the MH is an arc in the tube, some larger CF's can also trip arc faults, this can be a problem if you want to use energy efficient bulbs in your bedroom.