The RIvulatus Green Terror averages 6"-8" in an aquarium, but has the potential to be 10". Your 36 gal. Would work for a year maybe two but would eventually be too small. My 8" GT spent the first few years of his life in a 65 gal. Community tank with a Jack Dempsy, Bolivian rams, Odessa barbs and a list of others and did just fine. Be sure to start with a juvenile. They are easy to raise and a very active fun fish to watch.