system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If fish in an aquarium will eat until they die, how do fish in the ocean, or those same aquarium fish in the wild ever survive?
gpt
Overeating is not the real problem. The real problem is tha aquariums are small closed systems.

Fish poop, contains ammonia and ammonium both of which are poisonous.

The aquarium has filters and and bacteria that remove the ammonia and ammonium or convert them to nitrate and nitrite and then eventually to nitrogen which escapes as a gas. These systems can only handle so much.

When we increase food in the aquarium, there is more pooping, but there is also food left over to rot. Rotting food releases nitrates and nitrites.

Because aquariums are small, it doesn't take much extra ammonia, ammonium, nitrate or nitrite to overwhelm the system and cause the environment to become poisonous.

This doesn't happen in the ocean as readily because the ocean is a huge system. If you want to know how  the system works, you could Google "the nitrogen cycle".