Oh it's from a local show (in Cantonese) which is ~2x years ago. Here is the approximate content.
Supposed there is a merchant selling fish balls (in sticks). Then two people (A & B) buy one stick of fish balls. A gets a stick of 6 and B gets 5. B complains about it.
Merchant says: OK I will give you back one more. B: No I don't want one more. I want you to take away one of A's fish balls.
Since then all locals who watched the show would refer it as "fish ball mentality" and kept being used on local online forums/social media groups.
"crab mentality" was never mentioned anywhere (for me at least, until I see it here).