It's a decent chill game where you collect and grow fish. As you gain more money, you move onto more different types of fish and more tanks to grow them in. That's the whole gameplay loop in a nutshell.

Now there are different rarities of fish to collect (Common, Rare, Epic, Legendary, Mythic) and different color variants as well (Normal, Painted, Golden, Rainbow). All the achievements are tied to collecting the color variants of the Fish. (Collecting all from each species, each species painted, and each species gold.)

This is where the game gets annoying as you have a 1/1000 chance of getting a gold fish and even less of getting a rainbow fish. Most of the end-game, achievement collecting is just spam buying fish and selling them until you get the golds you need. It's okay.