chocolate fountain

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I have a chocolate fountain in the house. The first time I used it, I used the chocolate kisses that goes with the package. My friend who's a baker told me that I can use chocolate blocks that I can buy from baking supplies store and just use a double boiler to melt it. She also told me to add butter to thin it. My problem is that the two times I used it, it was either too thick or too thin. Is there a proportion I should follow? Also, somebody told me that I can use vegetable oil to thin the block, is that alright?