To catch the Loach, head to the northernmost cove during summer. Cast the Sinking Lure or Frog Lure on the lily pads and wait for it to bite. Landing the Loach is very difficult.
It’s harder to catch the Hylian Loach with the Sinking Lure since it sinks and gets stuck in the mud. Instead, make sure you have the Frog Lure from the Rollgoal mini-game. Now go to the fishing hole and go inside Hena’s. Make sure not to roll or else you’ll scare the fish or get into problems with Hena. Once you catch the Loach you’ll get a picture taken, which will go up on the wall. Ask Hena if you can go fishing alone. Make sure you always fish alone when trying to catch the Hylian Loach. Also make sure it’s summer by looking at the trees (they’ll be green). Go to the lilies, which you can find by going to the little indent on the map in the northern part of the fishing hole. Use your Fro Lure. The Loach looks like an Ordon Catfish but with blue eyes and red fins.
Find the room where you get the Boss key inside Zora Temple. There are two Hylian Loaches and a Skull Fish. The Skull Fish take up bait so be careful not to catch it.
Hylian Loaches can be caught in the following locations:
Fishing Pond by the Lilypads. Kakariko Graveyard by the Zoran Grave. Lake Hylia were the Poe was found. Use your senses and dig at any black spots. Up by the northeastern part of Hyrule Field. Use your senses and dig at any black spots.