Yes, the missing wall colliders is on me entirely. I forgot to implement them in the final build. My bad, really.
But sucking goldfish in didn't work? I play the game after every comment I read, to verify the mentioned issues (yeah, sorry about missing audio cues, btw) but that always worked for me.
Could you, please, try just once, getting close to a goldfish and using S-key for suctioning? The goldfishes also try swimming away from you when you get close.
If it works for you this time, then I am pretty sure the problem is with lacking UX that explains the mechanic fully. Like, having fishes light up when they are within action range.
thank you for playing, btw. I really appreciate it.