Generally it is the ID. Not very often is it referenced as the OD. Now, I am sure there are times when it is.
FW 90gal w/ 20gal sump
Current Stock - 14 Columbia Tetra/11 Serpae Tetra/16 Prisella Tetra/6 Yoyo Loach/1 Red Tail Shark/10 Giant Danio /2 Lazer Corys/6 Bolivian Rams/4 Otocinculus /6 Borneo Suckers/ 1 Albino Bushy Nose Pleco/ Flame Moss / Moneywort / Vallesaris / Java Fern...