We got our first ClownKnife when it was a baby. We raised it, initially, on trout pellets then moved on to live gold fish. The older of them (now two) Clowns was the only survivor of a fish tank catastrophe about five years ago. They are now 50 cm and 45 cm long. The older of the two is now 10 years old. They both live in a 450 liter tank alone and are right in the living room. Our guests are always fascinated by them. My brother and I like to watch them "hunt" when the lights go out.
I recently purchased a collection of Royal Clown Knives. A juvenile about 23 cm and an adult measuring 48 cm. I put the juvenile Clown with my pair of Chocolate Cichlids, a baby Silver and a baby Australian Arowana, all about 15 cm. The Adult is in my 380 liter along with a 45 cm Australian Arowana and some other fishes. The juvenile Clown shows little aggression, but the adult on the other hand is very aggressive towards the other large fish. He sometimes places himself in the middle of the tank and appears to bark. He makes a deep bark and than begins to display by shaking his body side to side. A friend of mine bred a pair of clowns many years ago in a 1500 liter tank. That was the only other time he's seen and heard such odd displays. I since had to divide the tank and keep the adult Clown alone. I think I would have lost my adult Australian Arowana if I had not done this. The Arowana has a long way to go before he is completely healed of his wounds from the adult Clown. I have a 690 liter that is nearly complete. Hopefully with the increased size I can keep both fish together without the divider.
I had a clown knife fish for about seven years before I lost him in a very sad tank mishap. He was a wonderful pet, beautiful to watch hovering or swimming back and forth across the tank. Mine had what I considered to be perfect markings: twelve evenly sized spots on one side and eleven on the other. I'd suggest waiting until you see a spot pattern in the fish store that you like, as some of them have irregular patterns. They are ravenous eaters, mine at about 35 cm length could easily down five or six goldfish until he had a huge lump in his stomach. Watching them catch fish is fascinating too as they creep up then suddenly lunge and open their huge mouth which creates a vacuum to suck in their prey. I would highly recommend them to anybody that doesn't mind the extra work of feeding live food.
I have a Clownknife also and I love it, although it gets a little too big too fast. In six months it has more than doubled in size and I am about to upgrade it to a 250 liter tank. It is a little over 30 cm now, I bought it at about 8 cm. But the way it swims is AWESOME. And it is a brilliant hunter, the way it captures then hides then comes back and does it over and over till all the fish are gone. Also, this fish does OK with my smaller firh. It has been living with Bala Sharks and Cory Cats and a Plecostomus since its first days here, and does not harm them at all. They are a little scared around feeding time but that is it. He also like to eat Brine shrimp with the rest of them.
My Clown Knife fish is one of my favorite fish. It is just under 60 cm and presently living in a 680 liter tank in my bedroom. The fish eats about three medium feeder goldfish every other day. I've had it for 4 and a half years now. My fish has given me lots of hours of enjoyment and will continue to do so.
The Clown Knife fish is the most aggressive fish I have seen, from territorial, to feeding time. The Clown that I own (about 40 cm) will stay put in its territory till feeding time, then WATCH OUT! Also, when light's out, it is the most beautiful swimmer I have ever laid my eyes on. A highly recommended fish to all large tank owners!