Tangs usually love Nori, so make a "voerplek" where it would be convenient for you to either net it, or preferably where you can install a fish trap (use a magnet and clip to hold the Nori)- then be patient for at least a week, perhaps two, while the sailfin learns to eat in this spot. Then, if you're lucky, you can catch him while he's eating.
If this does not work, you can try catching him with a fish hook - I'm serious, done it myself a few times, and the fish usually recover quickly without any permanent damage. Get the smallest trout hook you can find, then cut, grind or file the barb off, leaving only the smooth shaft of the hook. Tie the hook to some strong thread (or a trout leader), bait the hook, and just dangle it in the water. Your biggest problem will be not to catch any of the other fish in the tank, and not to strike to soon.
Enjoy the angling, but leave out the Klippies and Coke...
Hennie