Don't mean to steal thefishofdooms thread, however I wrote a clown knife article a while back and I'd rather not let it collect dust...
The clown knife fish is often encountered in aquaria, however it is kept most often by those that cannot correctly keep this fish. Not only does it demand live or frozen food and clean water, but it also requires a very large aquarium of at least 240-300 gallons (some large knives may require larger tank around the 400 gallon mark) . Clown knives are very large fish and can reach over four feet in the wild, however rarely get over two or three in the home aquarium.
Now for setting up the tank. Substrates should be sand or gravel, or anything thats not sharp and could injure your knife. Clown knives are naturally nocturnal and during the day will often hide (especially when younger, less so as they grow). Fish deprived of a hiding spot may get stressed and picked on by other fishes. A large PVC pipe will work, however it will eventually need something bigger to hide in. A large cave should do just fine, however take care to avoid stacking rocks on top of each other or they could fall and injure the knife. Because of their large size, heavily decorated tanks should be avoided.
Clown knives are warm water fish and should have a good heater, several decent filters (overfilter if possible), an aerator(not necessary if theres several filters), lights(clown knives live in areas where light is scarce, so the only reason for a light is so you can see them), and obviously water(a ph of about 6-8.0).
Clown knives occasionaly have stripes when young, but these fade with age. The black spots, however, dont fade (often the fish get more with age) When choosing your knife, aim for a specimen around the 6-8 mark. Examine the fish carefully for diseases and ask the lfs guy to feed the fish.
As for feeding, clown knives are predators by nature and will accept most any live food small enough to fit in the clown knives mouth(clown knives can open their mouth bigger then one would think)- and that includes other fish in the tank. Some people would rather not feed live food, and if so, clown knives will accept frozen food and possibly pellet, although it may take some weaning.
Tank-mates should be chosen with care. Although the knife may not seem to be aggressive towards them, knives tend to like late midnight snacks. On the other hand, overly aggressive fish may bully the passive knife to death(or eat it). Such fish include large Central American or African cichlids(though some will work), large catfish(i.e. Red Tails, Shovelnoses, etc.), arowana(works when both the same size) etc.
After you have the tank all set up, just cycle it, and add fish. Maintain weekly 20-30% water changes, and enjoy the fish!
Here's my old guy:
