This is an extremely naive, and incorrect understanding of how food webs and food chains work. They are extremely complex and if the "highest thing" dies, you have an explosion of prey species, which can have untold consequences to ecosystems and habitat. Read up on
trophic cascade.
There is nothing wrong with responsibly and ethically sourcing wild fish, the trick is having confidence that what you're being told is being done on the other side of the world at the time you're buying a fish is true. I'd like to see more certifications, almost a chain of custody, around the origins and methods used to catch wild fish.
You say you're a vegetarian, do you only eat organic fruit and vegetables? Otherwise the chemicals used to to grow and harvest your food have devastating affects on insect populations around the world. Which snowballs into larger problems both in the food chain and with pollinators we depend on to grow food for humans. Do you wear leather? Wool? Eat honey? Drink milk or eat eggs? All of those are derived from the exploitation of animals. As others on here are saying, you're initial question is innocent enough, but to say that X% of reefers don't care about the environment is just you stirring the pot in the worst possible way. The world is not white and black, and everyone is entitled to their own opinion. If you want to argue with other about why they "should leaveda fish alone" you need to have better rebuttals than you don't own the house you live in or aren't legally allowed to drive.