I wouldn't say you can't add another,I think as long as you have a few safe retreats for the new clown to hide untill they get use to each other(I could be wrong.)Go pick up one of those clear plastic tank dividers,that way they could see each other and not harm each other,you can get them at most fish stores for a wide range of sizes.
One will grow bigger and change into female once they're togather.
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Jim
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