Originally released on mobile in 2015, Koi is a relaxing little indie game about a fish who opens flowers and fights pollution. Imagine the opening stage of Spore crossed with Flower (which also recently came to Steam), and you've got a good idea of the basic concept.
Sometimes there are puzzles, like memorizing a pattern of flashing leaves to revive a dead branch so it stops blocking your way, or obstacles like a huge black fish that patrols between you and the smaller, color-coded fish you need to rescue. Each of those smaller fish—green, purple, yellow—can help you open flowers of the same shade that float on lilypads.
Although the threat ramps up slightly in later levels it's mostly a calm and meditative experience, one where you meander around the levels looking for hidden puzzle pieces and stars while the soundtrack plinks away and raindrops spread circles over the water.
Koi came out on Steam this week. The blurb says it will help you "Escape the realities of life and achieve a level of mindful satisfaction." I don't know about that but it's worth a look if you need to zone out and look at some chill fish for a bit.