Yup, I totally agree with Christmasfish. One goldie to a 10 gallon is plenty. You will be amazed at how quickly it will grow and how dirty your gravel will be when you go to clean it. They are just dirty fish. I personally wouldn't put another in the tank unless it is small, like a feeder goldfish that you wish to raise, and only if you plan on getting larger tank later. Technically, white cloud minnows would live with them, but only if they were large, or your goldfish exremely small. And a no-no to the tetras as well, they require tropical temps for their water.