I think its 12" but from what I remember reading they grow pretty slowly. I would say 75g would be fine for a grammodes, not sure about tankmates cause they are REALLY aggressive.
I cant remember who it was on here but they got amazing growth from their grammodes but that required a lot of water changes and a great diet. I believe he went from like 2 or 3 inches to 8 inches in less than a year.
Right now my grammodes is in the corner while my istlanum makes sure he stays there. Doesnt harm him just makes sure he knows whose tank it is. As they grow the grammodes will probly turn psycho but im selling the grammodes to hellman to pair with his female