Hey, Aquarist!

The manga centers on Tetsuo, a rough-looking guy with a secret heart of gold. In the aftermath of a summer festival, he sees two goldfish swimming in water-filled plastic, unceremoniously thrown in the garbage. He decides to pick up the goldfish and set them free in a nearby river, but is stopped by a beautiful girl named Kanade. Introducing herself as an aquarist, Kanade convinces Tetsuo to keep the goldfish. But it turns out that doing so is more difficult than it sounds.

(Source: ANN)