Shin Chimoguri Ringo to Kingyobachi Otoko

The "goldfish bowl man" is a man whose head is a goldfish bowl. When he sucks your blood, you turn into a goldfish. In order to stop him, Ringo, a.k.a. "Blood Diver" wearing his school swimsuit, and Haosuke, an ordinary high school student, take action.

(Source: Earth Star Comics, translated)

Note: The original story was published in a magazine that was shut down, and the manga with it. This is a continuation it received in Comic Earth Star.