It is true that we, as rational humans and moral agents, do have a duty of non-maleficence. We ought not cause pain to other creatures. However, defining pain or unhappiness or dis-utility is difficult in non-human forms of life (it even is difficult to compare pain between humans). The brittlestar has no centralized nervous system, and cannot experience pain in any human sense of the word. Certainly, killing a spider causes more displeasure to the spider than any trauma to the brittlestar. I'm not sure if our duty of non-maleficence extends to creatures this basic in structure.
From a purely utilitarian standpoint, we ought to weigh the consequences of our actions. If we introduce the brittlestar to my aquarium, it may keep my aquarium slightly cleaner and it will live a brittlestar life. I will, however, have a lesser opinion of the fruits of my favourite hobby, and every time I look at my tank I might get creeped out by the sight of it. Currently, nothing about my tank creeps me out. If I choose to dispose of the brittlestar, it may experience some amount of displeasure and I might find that a little saddening. My tank will remain brittlestar-free and I will not be creeped out by anything.
Weighing these two options with a utilitarian eye results in the second option seeming better. The pain of me, a rational human capable of complex emotion, outweighs the pain of the brittlestar, a small organism capable of no emotion whatsoever and may not even feel any pain.
I enjoy reefing. The first thing I do when I get home in the afternoon is look at my tank, and its the first thing I look at in the morning. I get a huge amount of pleasure from my tank. My tank is a happy place. It is beneficial for it to be creepy-free.
lol
