The appetite shift and no apparent signs could be indicative of internal
parasites. Heximita, round worms, and tape worms are a couple
examples..... Years ago I had many Angel deaths as you are
describing...I did some research and found that many breeders use
preventive measures against these common parasites with good results.

Thank you, that might be it. I'll see if I can find similar stuff here in
Denmark.

Do you have any idea if there is a suitable quarantine period after which
there should be no more parasites left? Or can they survive at at "low
level" in the other fish?

I would be sad if the treatment killed off the shrimps.

rgds

Martin