They exist, they're just not legal in the US (and Canada obviously). Europe is awash with them to the extent that we hardly ever need antibiotics.I wish we had an alternative to antibiotics for fish, as there are issues with misuse and overuse in untrained or unskilled hands. But they don't exist.
That doesn't mean we're not still guessing when we treat our fish, and we still only treat for the obvious ailment when most times there are several, but our available treatments are much more bio-friendly.