Hi and welcome to AQ.
Are you talking about circus clowns (land dweller in a big tent), clown loachs (freshwater), clown killifishes (freshwater), clownfishes (marine) or clown wrasses (marine)?
If it is the 1st one, ask that guy to visit the doctor.
For the next two, observe them carefully. For all you know, it could just due to what they ate before you bought them, e.g. daphnia. They may be getting stressed out by the initial stages of tank cycling if your tank is not heavily planted. The effect of NH3/NH4 and NO2 varies from species to species. Read up on your fishes and the ways we cycle a freshwater tank.
For the last two, you really shouldn't be putting fishes into a marine tank so soon. The cycling process is very likely kill all your fishes. Try to get a friend with an established tank to keep them for you until your tank is ready.
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