Goldfish fry

My Fantails spawned about 3 weeks ago & I moved the pots with the eggs in to
a new area that has no adult fish.
There were hundreds originally, but now there only seems to be about 40 or
so.
I have found a lot of dead or nearly dead fry in the last week. We have had
some very extreme weather here in Sydney & am wondering maybe the heat
killed them? (For two days it was over 38c).
The ones that have survived seem to have tripled in size, so could it just
be a case of 'survival of the fittest'?
Ta
Pat