The tips in the link look pretty good. I would add one other alternative - try using a large clear plastic bag rather than a trap or a net. Just anchor it in place on the bottom, then herd the fish in. (The fish don't tend to detect the bag's presence as easily as a rigid object like a glass or cup.) You can then close it with your hand. However, I suspect you'll have to try a few different methods before you have success - it will depend on the species of fish you're trying to catch and the layout of the tank, and even then there will be a lot of individual variation. Also Centropyge angels are among the trickiest little fish to catch. One ichthyologist has even suggested this is the reason certain surgeonfishes mimic Centropyge angels - they are difficult prey to catch, so predators tend to avoid them (as well as their look-alike mimics).
I'd like to relay one story regarding my efforts to catch dottybacks in an aquarium - which are probably even more elusive than Centropyge. Twenty-five or so years ago when I first started doing research on the taxonomy of dottybacks, I had a large tank with a series of mesh cages which contained what I believed to be two different species of the genus Ogilbyina. (The cages each had one fish, which were separated so they wouldn't kill each other.) I was heading off on a research trip for a couple of days, so tried to do a little work on the tank before catching my train. While messing about moving things around in the tank, I bumped the cages and two dottybacks escaped, one of each of the two species. I spent an hour or so trying to catch them (even using plastic bags), but eventually had to give up in order to catch my train. A few days later I returned, expecting at best to find just one dottyback in the outer part of the tank alive. However, both were still alive; in fact they had formed a pair! It turned out they were the same species after all (Ogilbyina novaehollandiae). This led me to research on colour variation in the species (there are five distinct colorations, associated with age and sex), as well as giving me an appreciation of colour variation in other pseudochromids. Without the insight from this accident, I may not have made much progress in sorting pseudochromid species out.
Tony
