I didn't even think about walleye. It does look a bit like a walleye fry but I doubt it is, I'll let some others chime in but I'm 99% sure that's a brook stickle. Don't be sad about it though, Sandwr species (Walleye, Sauger, Xander) aren't good aquarium specimens anyway, they just kind of mope around in the corner all day. They're beautiful fish to keep just not active.