The reference from dealer installs says the vent is a manual valve at the highest point of the tank, with the line running up and ending above the waterline. That lets you bleed trapped air when the tank floods, since air near the transducer path hurts the picture. Make sure the tank top is the true high point, with no pockets that can hold air.
I don't have hose size, fitting type, or the exact valve spec, so check the Furuno installation manual or ask your installer. Since the tank is a below-waterline structure, the vent routing, valve and any penetrations should be done or signed off by a
⚓ 2 Tight lines