Goldfish and Fantail sitting on bottom not feeding - HELP

A little history
I have a 36x12x15 fish tank with a fluval 2+ filter and air stone, which
had 3 large goldfish and a fantail in (all 4 years old)

1 of the large goldfish unfortunatly died shortly after moving house - I
found that the power had gone off and the airstone had stopped working
- it died during the night

All was well for a while..

Now I recently purchased 3 more young fantails (grey colour) and added
them to the tank
I seem to be having problems since

One of the goldfish developed 3 white "spots" on its mouth and seemed
less energetic than normal
I assumed this was a mouth fungus so I put some fungus treatment into
the water

Then the 2 large goldfish and fantail started just sitting on the bottom
They have been like this now for a week - they hardly move, just lay on
the bottom and very rarely eat
I just soaked some food in water for a while and dropped it in and
they've completely ignored it

A few days after they stopped moving around one of the new fantails was
found dead - unsure why but its been removed from the tank

The 2 other NEW fantails seemed fine swimming around happily and they
still feed although one does seem to have some red patches on its tails..

Im at a loss as to what to try - I noticed their poo has had a white
stringy bit too it and has been floating around - I cleaned most of it
out but it didnt help

I also removed 60% of the water from the tanks and replaced with fresh
and that still hasnt helped

Ive been treating them with swim bladder treatment fluid every 48 hours
for 3 days and still no change

The large fantail and the 2 large goldfish are all huddled around the
filter on the bottom 24 hours a day now and its really worrying me

Sorry for the long post - Any help would be very much appreciated

- Phil