Fish are everywhere in Animal Crossing: New Horizons. You need to work on capturing all of them if you want to see a full museum, and some of them vary in what time of year, time, and how you capture them. If you’re looking to catch the stringfish, you need to go to a particular location.
The stringfish will spawn from December to March in your game, from 4:00pm to 9:00am, meaning you don’t want to wake up and go after this fish. It’s likely something you hunt for later in your afternoon. The only way to capture this fish is to locate it on an island with a clifftop, which means you need a ladder. For those looking to unlock the ladder, you receive the DIY recipe when three new residents want to join your town, and Tom Nook gives you the quest to make the bridge. When you place the second house down, Tom calls you to provide you with a ladder. You now have access to cliff sides.
Now that you have access to the cliffs, you can scale them and try to find one of the pools of water resting on it. You want to make sure you bring fish bait to spawn one of the larger fish shadows, and a higher quality fishing pole to capture it. Throw the fish bait into the water until you catch sight of a larger fish shadow in the water, and from there, throw your rod. Once the bobber goes into the water, reel it in to see what you capture.
Fish come from a variety of different locations in Animal Crossing, so make sure to pay attention to the shadow in the water. Once you capture the stringfish, you can sell it for 15,000 Bells.