What are you testing ammonia with? A 1 bottle test kit or 2 bottle
salicylate based ammonia test? If only 1 bottle, that is why you're showing
ammonia, if 2 bottle, and you're still getting ammonia readings, add more
Ammo-lock, or the other brands RTB mentioned.

Two bottle. Aquarium Pharmaceuticals. The literature says the test
will still show positive even when the ammonia is inactivated. So
when I treat again, I figure I have to treat for the difference
between the last reading and the current reading.

Before all this happened, there were 17
goldfish in there, and the water chemistry was perfect.

17 goldfish in approximately 275 gallons (assuming you didn't fill to the
rim?) I'd say they were over crowded. Even in the best conditions it is
recommended to give goldfish 20 gallons/fish. More if you want them to grow
well.

You're probably right. We only put 12 in there last December, so I
don't know if we had spawning and rapid growth or whether we picked up
some fry or eggs in the plants we bought. Interestingly, several of
them were young koi, which we had never put in there in the first
place. We had only planned for the fish to be in there a little
while, but there have been problems with getting the pond built.

Aeromonas hydrophilia is a 2ndary infection, and the stress of over
crowding can weaken the immune system. Did the fish specialist do a scrape
and scope to see if any parasites were involved? The salt, if added over
several days to get to 0.3% shouldn't have killed the filter, nor should
have the antibiotic food.

The vet did rule out parasites. As for the primary cause, we had an
exterminator to the house a few days previously. It wasn't a windy
day, and he only sprayed the perimeter of the house, which is maybe 10
feet or more from the tank. But perhaps some got in there, weakened
the fish, and then they got sick. The timing worked out in a
suspicious wat, and the vet thinks that is what happened.

As for the biofilter crash, we think it's a number of things, not the
least of which is that I didn't remove the plants soon enough after we
increased the salt level, and I think there was massive cell death of
the plants under the water, even though the tops still looked green.

Koizyme works really well on lowing the Aeromonas and Pseudomonas bacteria
in the water column, but it is important that the fish have a good immune
system to also fight those buggers off. ~ jan

The problem was the fish wouldn't eat it. I think they were too sick.
And yes, probably too weakened even before they got sick. :-(

Joan