Fishing: There's more to it than you think
Fishing is a great way to increase your professorial level without spending activity points on food. Sometimes on Sundays there are events where it is rare to catch fish that you can sell for cash. But it is important to fish wisely, which means that you should not necessarily catch the first fish that bites!
When a fish is infected, an icon will be displayed indicating what type of rarity the fish will be. If you use expensive bait to catch rare fish (for example, on a special day), you will need to wait for the purple, gold, or rainbow fish icon to appear to make sure you catch the fish the most from that.
However, you risk losing your bait after the third hook, sometimes after the fourth, if you're lucky. So try to come out while you are ahead.
How To Scale The Rarity:
Blue -> Red -> Gold / Rainbow / Purple (Special Fishing Events Only)
And remember, the better your timing is in the mini-game, the better the fish! Anyway, it says in the game.