If you fish under the bridge with any rod on Route 119 during the day, you will always get a Feebas. They made it pretty easy to catch this fishy. It was near impossible to find it in RSE and the 4th gen games.
This also makes it pretty easy to chain fish them. I managed to catch a shiny one after a few attempts at chain fishing.
Wish I knew this before I spent about 2 hours fishing for it. Do any other Pokemon have similar tricks?