Most of the snowflakes I’ve seen at stores have been confined in some way, usually in what looks like a little cricket carrier. The store where I got mine says they do that so they can easily remove the eel vs trying to net them in a big tank.
I haven’t had my snowflake for very long, just a few months, but my understanding (and my observation) is that they do very little “stretching.” Mine likes to thread through and around the rocks in his tank. The only time I see him out and about is when he’s hungry, and that’s usually my sign to feed him the next day.
What I’m getting at is that I don’t think the concern would be muscle degradation so much as just general poor health from being at a PetCo for so long.
I will say that of all the fish to not impulse buy, eels would be near the top for me. I did a ton of research before hand. At the very least you can’t bring an eel home to a tank that isn’t locked down. They also get fairly large. Pushing three feet at the high end and about as fat around as a man’s wrist.
I understand the impulse to “rescue” animals, but as Jekyl said, that only benefits the perpetrators of poor animal husbandry.