I caught the 18-pounder (as Adult Link) and I still won the Golden Scale. Only took me about 40 minutes. The trick (like fishing IRL) is just patience. Don't get mad about it, try to have music or a movie going on in the background as you fish. If your aim is right, the Hylian Loach will take notice of your line. It tends to lose attention pretty easily, so don't get mad if it does. I found a good strategy was to wait until it stopped wagging its tail and then shift the line with the joystick. Keep doing this, and eventually the fish will lunge at the line. It's tricky to reel it in... the trick, I found (I'm not sure if it works) is to pull the joystick in the direction the loach isn't moving (so if it's pulling left, you pull right, if it pullls away from you, pull it to you). The Loach changes direction a lot. Just keep at it and you'll get it in time.