I had to catch a Bangai last year and it took a while but this is how I did it.
I bought 3 different nets, 2 large ones and a smaller one(get the ones with black mesh so it can't be seen at night), then I chased him around the tank a few times with a piece of flexible tubing attached to a piece of rigid tubing to watch his patterns. The flexible tubing lets you poke into caves and crevices where the rigid won't go. I started to figure out if he was in a certain hole, and I poked into that hole, he'd pop out and circle around to a different spot. I did this with the lights on until I figured out his patterns. Then I waited for nightfall with the house completely dark and put on a headlight w/ a red beam so I could see in the tank. Place your larger black nets in a couple of strategic spots where your cardinal will circle around, then start chasing him with the tubing. Use the smaller net to help corral him when he gets close to the larger nets. Sooner or later he'll swim right into one of the larger nets and you just have to be ready when it happens. I've actually used this on several different fish where a fish trap wouldn't work for whatever reason and it works pretty well. Good luck!