Biofilter crashed

I have 9 goldfish living in a 300 gallon stock tank till my permanent
pond can be built. They've been doing fine there since December. Had
a normal nitrogen cycle at first, not too bad, then water chemistry
consistently checked out great. In July, several fish died. Necropsy
showed aeromonas hydrophilia. Consulted fish specialist, who had me
get salt concentration to 0.3% and give the fish 5 hour baths in
Baytril (outside the tank). This was after trying medicated food,
which the fish wouldn't eat.

To make a long story short, the plants died, the uneaten medicated
food fouled the water, the antibiotics killed the biofilter, and the
ammonia level went through the roof. We had nowhere to keep the fish
during the emergency except for a few hours in the 30 gallon container
they had their "baths" in. So we did several 100% water changes and
cleaned everything and started over. Followed up with daily or every
other day partial water changes because the ammonia levels kept
creeping up. A week or two later, there's no end in sight. Lately
we're doing 50% water changes daily. I'm trying to keep the pH in the
range of 6.8-7. Using Ammo-lock as needed. Dechlorinating the water.
Keeping it well aerated. Keeping the salt at around 0.1%. Checking
the chemistry several times a day. Adding good bacteria. Putting
zeolite in filter and skimmer. Recharging the zeolite periodically.
Water temp stays in the range of 68-72 degrees.

The ammonia level is creeping higher and higher despite all this.
Today it was around 5 ppm *after* the water change. Looks like I'm
gonna hafta do another major water change, maybe 75% or more.

So far, there is *no sign* of biofilter coming back. Not even a hint
of nitrites.

I don't think the fish are overcrowded. They range in size from, say,
2 inches to 6 inches. Before all this happened, there were 17
goldfish in there, and the water chemistry was perfect.

Any suggestions? Thanks.

Joan, Desperate to Save My Fish