Goldfish keep dying

I have a 55 gallon / 220 L freshwater tank with 12 goldfish in it (mixture
of blackmores, shubunkins and comets). Recently they started dying at the
rate of 1 per week, so we end up replacing 4 a month. The oldest are 2 years
old.

We have had the water checked for everything (nitrates, nitrites, pH, etc)
at the fish shop and everything is fine. We tried doing a salt bath, monitor
pH daily and have ended up doing fairly frequent 25% water changes. It's
been pretty hot lately but water temp is OK (28 deg C or less).

The fish look fine (scales OK, no bloating, no weird colouring) other than
every few days one of them starts looking depressed, its tail fins hang down
and it starts living on the bottom of the tank. Within about 4 days it's
dead.

What could be going wrong? This was an established tank and then the fish
suddenly started dropping like flies...

Engram