Melted chocolate is very temperamental, so rich couverture chocolate, which is high in cocoa butter, is commonly used to ensure consistent flow. If the cocoa butter content of the chocolate is too low, an additive must be mixed in to decrease viscosity. (Vegetable oil is most commonly used to do this.) But even couverture chocolateunless specifically designed for fountainsoften still requires an additive to make it flow smoothly. Because of this, it is highly recommended that chocolate formulated specifically for fountains be used to avoid the need for the addition of vegetable oil, as the oil gives a slimy, gritty taste and texture to the chocolate. Few chocolate fountains are capable of melting chocolate directly in the basin, so chocolate is typically melted in a microwave or double boiler before pouring it into the fountain.
The fluid dynamics of home chocolate fountains are particularly problematic, since the gravitational forces are much lower than the viscous forces; this means that small chocolate fountains need a very high proportion of vegetable oil. There is a trade-off between looks and taste: if perfect smooth flow may be sacrificed, it's possible to use non-oily chocolate.
A cheaper alternative to couverture chocolate is chocolate-flavored syrup, also called "chocolate coating". Chocolate coating is already in liquid form and costs much less than couverture chocolate - although many would argue that the extra expense of gourmet Belgian couverture chocolate is more than worth it. Another alternative is to use dark chocolate, such as 70% cocoa chocolate. This contains much less sugar, so it isn't so sticky; a small quantity of hot water may be added to thin it further.