Black Shuck wrote,
He agreed, that what killed them, was a parasite
infestation, rather than a fungus infection...

I didn't think you ever saw anything (parasites) on the fish(?) I took
a week trip to Florida durning that time and it was all over with by
the time I got back. With no visible symptoms, but had uncoordinated
swimming just before dying, my thoughts were along the lines of a
fungus called Saprolegnia - invades the gulls causing suffocation...

He also said, that because
my tank has now been empty of fish for the last week that any parasite
would have died, as there was no host fish to breed on.....

Hmmm - not so sure about that. What could have died however, is most of
the bacteria within your filter (no food for a week). You might want to
add a drop or two per gal. of pure ammonia to the tank. After a few
hours, if the filter hasn't metabolised the ammonia and nitrite, you
may have to 'feed' the tank a few drops of pure ammonia every day for a
few days to build the bacterial colony back up enough to support a
bio-load.

The guy at the shop suggest "chucking" some protozin in,
half a bottle, and leaving it 1 few days, just to be sure!!
Does any of this sound like good advice?

No since in treating an empty tank - buy the fish and if need be, put
them through a quarantine preventive treatment. ............... Frank