Heh, I actually checked about the goldfish. You can get them on the shopping channel! Want to know which day? October 2nd, the one you just missed. I know, it "amused" me too.
Really, you're not doing anything wrong with the fish. It's just chance which ones you get. They encouraged you to do it on a sunny day because the usually good fish, notably the Huge Fish, only come out on rainy days at this point in the game. But you can get the crappy red goldfish any day. So if you want to fish again, for God's sake, don't do it when you could be raising an S.Link. Wait for a quiet night when Nanako and the increasingly less naughty nurse are around, for example.
To further enhance your chance, when you fish, watch the speed of the ripples and the vibrations on your controller. In general, the bigger the fish, the longer the gaps between ripples/vibrations and the stronger the vibrations. A goldfish's will come very quickly. If you get one that feels longer, just press the circle button before the fish bites to switch to a different one. I know, all of this is tedious, but the game didn't anticipate people making videos of it and refusing to reload if they get fish they don't want.
