Take several fish of similar species and size, and put one of them in a dirty, far-too-small environment. It would be easy to tell through objective observation which one was unhealthy and stressed. That does not require me to project any human qualities onto the fish. I don't have to say the fish is "suffering;" "suffering" implies emotional and psychological baggage that perhaps fish don't have. Regardless how the stress is subjectively experienced by the animal, you still have to justify purposefully subjecting it unnecessarily to stress, disease, and a premature demise. Therefore the question remains, what criteria do you use to determine which species are okay to treat in such a manner, and which are not?