Are we sinful if we do not feed ornamental fish or clean their water, and what is incumbent upon us to do regarding them, fearing that we might be like the woman who entered the Fire because of a cat?

The texts of the Sharia indicate that a Muslim earns reward by showing kindness to everything, and animals are included in this. So, if you are able to feed the confined fish and clean its tank, then do so, seeking the reward. If you are unable to, then you must consume it or release it. You should also advise your employer regarding the impermissibility of confining an animal without feeding or caring for it.