This question is not just Victron related, but hopefully people around here are more educated than I am on this topic 
My main electrical circuit on board is built around a LiFePO4 battery with Cerbo GX, 2 MPPTs, Orion XS and an Inverter Charger with Smartshunt as DC Energy Meter.
This circuit shares a common ground with the starter battery and therefor the hull of my boat.
In the front of my boat is a bow thruster that has its own battery with short + and - cables to the thruster (<50cm). Of course secured by a 250A fuse.
The Smartshunt has an AUX port that I used to have on the + of my starter battery, but with the addition of the Orion, this is redundant as the Orion already shows the voltage of my starter battery.
So I would like to use it on the + of my thruster battery, so I know what’s going on and when I need to charge it, as - for now - it is not connected to the alternator or any other DC charger; I have a Victron Blue on standby for this.
Because there is no common ground, the AUX will not work right away. So I was thinking of running a low current (2,5mm2?) wire from the front to the ground bar in the engine room.
How can I do this safely? AI tells me to add a small fuse in this wire, near the battery, to keep high current from the thruster going through it in case of a malfunction in the front. Good or bad idea?
What other way could I do this other than using 50 or 75 mm2 wire (expensive!!)
Thanks for any insights.