A little follow up

Well as some of you read a week or so ago I was concerned about my fish
sitting on the bottom

I can unhappily report all of them are now dead with the exception of
one large goldfish which looks to be beyond recover too

3 days ago they were still the same sat on the bottom of the tank and
the last 2 little fan tails I bought had both died
Red blood patches around there boddy and tatty looking tails
Then yesterday my 5 year old large fantail was floating on its side on
the bottom - dead

Now today one of the large gold fish (4 years old) was the same - laid
on its side on the bottom very much dead and so were my final 2 little fish

That leaves me one large goldfish which seems to have white fluffy stuff
on its head and still will not feed

*sigh*

And the pet place said my water only had small amounts of nitrate and
nitrite and no ammonia

So I have no idea what killed them

What is the best way to clean the tanks out and stones,plants etc in
case there was an infection - so I can start again..?