dont do it for 2 reasons !!
one being mangroves are trees and over a bit of time the roots can and will wedge between the seems of the glass and cause it to leak
two the puffer will never alot it to grow ! would eat it or bite it just cause
also people mangroves dont have to be out of the water to start growing
if the tanks was acrylic ya could plant the seed at the bottom of the tank and it will reach and finally grow out the top of the tank with out any issues
most think that the top of the seed has to be above the water line this is not true at all