10-15 gallons will be too small for goldfish, unless perhaps if you kept just one. They need a lot of room. Also keep in mind that they need more filtration than regular fish, as they produce a lot of waste. Loaches might get too big for an aquarium that size, too. If I were you, I'd look into getting a nice little school of white cloud minnows. They would work nicely in a tank that size.
Hope that helped, and good luck!