Does your goldfish show any physical signs of disease (besides the obvious lethargy)? Since you did witness your goldfish swimming to the surface a few times, there’s either not enough oxygen in the water or something is preventing your goldfish from taking in enough oxygen (some goldfish diseases can affect the gills). To boost oxygen levels, you can try using an air stone (attached to an air pump with airline tubing). All of these items you can get at your local pet store. A little aquarium salt can also help reduce stress (you can read more about aquarium salt in this guide here ).