In English, Zeisses, probably. In German it would probably be Zeisse. There is no reason for a Latin -i ending.
There are actually cases for the declension of Zeiss, e.g. in the following absurd German poem from the 1910s by Christian Morgenstern:
"Ein Hase sitzt auf einer Wiese
im Glauben, niemand sähe diese.
Doch im Besitze eines Zeißes
betrachtet voll gehaltnen Fleißes
von einem nahgelegnen Berg
ein Mensch den kleinen Löffelzwerg.
Ihn aber blicket wiederum
ein Gott von fern an, still und stumm."
("A rabbit sits in a meadow/believing no one sees him./However, a man with a Zeiss/filled with suppressed eagerness/watches Little Big Ears/from a nearby hill./And him in turn/a god silently watches from afar." The original German is a lot better than my rather literal translation.)