Yes, it is Alternanthera. But it's A. ficoidea, a non-aquatic.
It's been like that for many years. Might just be because it takes the terrestrial stuff longer to die than a true aquatic that isn't happy.Thank you Cavan Allen.
I'm a complete newbie at this, but I'm surprised (and a bit bothered) at how common it is for pet stores to sell terrestrial plants completely submerged in water, as if they are aquatic plants. It seems like deceptive advertising.