It's not a fishing game. You need to gain weight to move down. It's in the initial tutorial text. You have to suck in the goldfish by getting close to them, so as to gain weight and sink deeper or drill-kill piranhas before they kill you.
Hey there Ham! I have just uploaded an updated version of the game with fixes that (hopefully) translate well through itch. Plus, a brand new lock & Key mechanic and 3 new short levels. If you get time, do check it out. I'd truly appreciate it.
