Fish antibiotics

I was given tetracycline when I had pneumonia as a kid. My fish got tetracycline when they got ‘ich’. (In looking it up, tetracycline does not appear to be a treatment nowadays, and ‘ich’ does not appear to be something one would fight with an antibiotic.)

Question: What is the difference between prescription tetracycline for humans and tetracycline intended for fish and other animals?

(Warnings about self-medicating, side effects, etc. go without saying.)