There's a difference between sentient and sapient, qv. A fish doesn't have the circuitry to experience foresight and long-range planning, therefore imagination, long memory and regret. What is, is, and he handles it very well. He learns, he accumulates experience, he hunts, he fears (briefly), but he does not, in the absence of a shark, fear being eaten by a shark. It's very to-the-moment, I suspect, and feeling safety in a school (gee, wow! we're big now!) does not mean he'll pine extravagantly when he leaves it because it's safe now.
I think rather than projecting our emotions onto the fish, while it's humane and human to try to apply empathy, which often guides us to kindness---it's instructive and broadening for US to try to realize that they're their own kind of creature, quite wonderful in their adaptation to a universe fraught with dangers that, with our imagination, would scare us silly.