I have a clown knife aswell, I love the thing.. They are hefty eaters though, I agree with everyone else, get a good filtration system.. Also I installed moon lights on my tank so I can watch the nocturnal feeding..
I have a pair of clown knives in the foot long range....You're looking at a hefty food bill as everyone pointed out. My clown knife tank is filtered by a five gallon bucket full of quilt batting (the tank is metal w/liner so it's internal) run by a pool pump.
Since mine were give away fish I just ran out and grabbed a handful of quilt batting from my pond (koi/goldies) filter and it was more than enough to satisfy the filter needs of the two clowns and the adult JD that came with them.