Her vitriol towards what Firo assumes is an innocuous fish only reads to him as more denial. He wouldn't put money on it at this point, but he's starting to wonder if she really does have some small amount of affection for the thing.

He shrugs and takes a can of what look like string beans out of a cupboard. "Don't know about useless. If it gets big enough, you could probably eat it."

The only 'pet' fish Firo ever saw in the tenements were the occasional Christmas dinner carp dumped in the bathtub and allowed to grow until the day of their doom. Perhaps that's what Hope intended with this lady's little thing.