Recommendation of keeping any tetra is no less than 5 to spread out the aggression. 180 is large enough to house 6 or more of these large tetra. There are 2 species of chalceus, erythrurus or macrolepidotus. Which species do you have and do you have pics?
Chalceus are a little different than most "tetras." They don't really school as adults; they tend to be pretty territorial, and at best, form loose associations. Unless you're keeping a phenomenal number of them, you're going to have a lot of aggression -- keeping a single fish is fine, and keeping three in a large enough tank is also fine. I wouldn't worry about your numbers too much -- if you start seeing aggression, thin them.
True, Chalceus are more like cichlid than community tetra. They don’t shoal but stake out territory, and stay motionless except feeding or attacking one another. So you have to manage their aggression by crowding like Mbuna or just keep one. But unlike Mbuna, they are only aggressive towards its own kind. I keep both species of Chalceus, one each in a 75 and 125, and they are totally peaceful to other fish.