Any game that has fishing in it is automatically a good game. Fire Emblem: Three Houses in one such game. In the game, fishing is one of the free time activities you do without spending activity points. If you have enough bait, you can fish as much as you like. You can buy bait from the Eastern Merchant in the monastery, plus you can randomly find some hidden while walking around.
Each piece of bait has a star level and affects what kind of fish you can catch. Once you select your bait, the fishing minigame begins. The minigame itself is really easy, you just need to time a series of Quick Time events until the fish is caught. Timing your button presses perfectly will cause the fish to lose stamina. Missing the timing will make you lose stamina instead. Get the fish stamina to 0 and you catch it.
There are 6 types of fish, with rainbow fish being the most rare and valuable ones. Among those 6 types, there’s nearly 20 different fish to catch. For example, the Fodlandy is one of the Rainbow fish. You can only catch this fish using the Herring Bait and it sells for a very high price. But if you’re only using a cheap bait like the Earthworm, you’ll only be able to catch low level fish. But what do you do with the fish?
If you’re just casually fishing, there’s a lot this activity can do for your playthrough. Firstly, every fish you catch will get you experience to increase your professor level. For example, a low-level fish like the White Trout will get you 10 exp while a Platinum Fish will get you 40. Once you’ve caught a bunch of fish, you can sell them all for a ton of gold. Most of the fish will net you between 10 and 100 gold while the Rainbow fish will get you more than 1,000 gold each.
Lastly, fish can be used as part of cooking recipes. In general, fishing is a very worthwhile event since it just requires a single bait and has so many benefits. Go forth and fish as long as you like.