The question is how do you define ethics and what is ethical?
Ethics is fundamentally a human construct, a social contract that we voluntarily agree to abide by. Shared values on what is right or wrong, and what is and isn't ethical varies from culture to culture, religion to religion, person to person.
Something one learns quickly studying nature, is that ethics do not exist there. Predators eat prey alive, lower lifeforms use higher lifeforms as walking incubators and nurseries, or even make their homes in them.
To me what is ethical in the context of the hobby is to strive to avoid causing unnecessary harm. Habitat destruction and unsustainable practices benefits no one. When It comes to corals, i prefer to buy maricultured whenever possible, which incentivizes responsible stewardship of the reefs.
As for the ethics of keeping fish? Every day in the wild is a battle for survival and unless the fish is an apex predator, it will most likely meet its end in the jaws of one. I won't throw out the clichéd excuse of "we offer them a better life", but I do believe what we offer aquarium fish is "no worse" than what they face in the wild.
At the end of the day we are the dominant species on this planet, be it by the grace of god or by evolution.