Thusly "Gill Passman" gillspamattaylorpassmanspam.co.uk Spake Unto
All:

They always die during the night?

They always get ill during the night....they die later either in the next
day or the following day/night

OK, that narrows it down a bit.

Two obvious alternatives are oxygen depletion and attacks by other
animals, e.g. some catfish, some crayfish... I had otocinclus which
attacked and killed fish during the night some while back, for
instance, and oxygen levels drop during the night.

However, you've listed your fish, and you've not got any fish which
should be capable of attacking other fish, and fish recover quickly
from oxygen shortage.

The main remaining possibility I can think of is flexibacter disease.
It is extremely virulent, and may kill fish so quickly that they never
develop any symptoms. The most resistant fish will, however,
eventually develop symptoms: white fungus-like growths, and ulcers on
their bodies, often around the mouth.

But until you see any symptoms, flexibacter disease is just a wild
guess based on the absence of evidence.