I don't really get it when people say not to do something to a fish because it's "not natural"... There is nothing natural in keeping a wild animal in a glass box.
But back on topic, if you intend to cut your arowana fins, gills or tail, and I believe the OP is talking about an Asian Arowana, I would suggest strongly that you sedate it first. A fish, especially a big fish, out of water, will flip all over the place, making it really hard to give it a clean precise cut. There is also the risk of the fish injuring itself, broken pectoral fins and what not...
Also note that Transmore is illegal over here, at least in Canada. A good alternative is a product called Asian Stabilizer, by Ocean Free, easily found on Ebay:
http://cgi.ebay.com/150-ML-OCEAN-FR...0?hash=item2e9c4e032b&_trksid=p3286.m20.l1116
Ordered it a few times, it works wonder. And excuse my spelling mistakes it there are some, I am French Canadian...