I know this is not what you are wanting to hear, but you need to take the clownfish back. The only thing that you might be able to cycle with is damsels, but I wouldn't even recommend torturing a fish for that.. here's what you do...
Go out and buy some live rock.. get some fresh shrimp and cut it up to cycle the tank.. make sure you keep track of water parameters and settings... Like the guy before me said, a sterile tank is not a good one. It will take some time, but don't rush it.. rushing it only leads to other problems... such as diseases.
Wait approx.. 4-6 weeks.. then check to make sure your nitrates/nitrites are 0, make sure your ammonia level is also 0.
Some things to consider.. so you don't make the mistakes I did.. don't get a cheap skimmer.. do not waste your money on that. I was using a piece of crap skimmer for 5 months.. and my terrible water quality led to an outbreak of ich.. and if you know anything about aquariums, ich is bad stuff.. get a good skimmer.. since i got my ASM G2 for my 75 gal, my fish are finally where i want them. Good luck..
and remember, if your LFS told you to use clownfish to cycle a tank.. that just tells you how much they know about the hobby.. so don't listen to a damn thing they say.