In Which I am Dissed by Tropical Fish

For the Mermaid Girl's birthday, the Renaissance Woman bought her a pair of fish. Black-skirted tetras, to be specific. About the size of goldfish, but slightly more exciting-looking. And they've sat on her desk ever since, in their small pink terrarium, swimming around in circles around their little pink castle, and occasionally hiding behind the heater.

They're pretty low-maintenance, as pets go. Every couple of weeks MG and RW scoop out half the water and clean the tank, and twice a day MG feeds them, flipping open the little trapdoor at the top of their tank and carefully sprinkling in just a few flakes of the green flaky fish stuff. They've learned that the trapdoor flipping means food's coming, so they always dart right up when she opens it and suck up the food and then swim around excitedly.

But here's the thing: every once in a while, MG asks if I'll feed the fish for her. And when I feed them, they never jump right up. Even when I make a big deal out of flipping open the little door, and even when I crumple up the flakes especially enticingly, they just swim around lazily, sort of like, "meh, well, whatever, I might grab a bite later..."and then maybe after a few seconds they'll drift up and take a nip or two of food, just to be polite, I guess.

So my question is: can it possibly be that they can tell the difference between me and MG, and that they like her better? I mean, aren't we all just big blurs to them? And, if they like her better...WHY??