Amyloodinium (Oodinium) upon adding new fish
I have a saltwater aquarium (240 l) and added new fish, these seem to have introduced velvet disease leading to my entire fish stock dying within the last days.
independently from the original question: I urgently recommend keeping new fish in quarantine for approximately 4 weeks if possible as to prevent this kind of thing. This applies especially in saltwater aquariums, because using treatments against diseases is possible only to a very limited extent due to the invertebrates)!
I would now like to know when I can add the first new fish.
You should wait for about 4 - 6 weeks.
And if I have to add a treatment first that killes possibly still present parasite strains.
No, but UV filtration is advisable.
At the moment, there are 1 cleaner shrimp, 1 brittlestar, 1 tubeworm and various corals left in my aquarium. Is it possible that these animals will die as well?
Somewhen these animals can and will of course die, but not in connection with Amyloodinium (formerly Oodinium) - this is a mere fish parasite that does not infect invertebrates.
And can I hold my retailer responsible since he sold ill animals?
This might be possible if you can prove that he knowingly sold you ill animals and you are trusted or can prove that you had no chance of discovering/noticing a disease in the retailer's tank - otherwise probably not. However, this is to be considered just an opinion - only a lawyer can give legal advice.
Quite unlike a widespread misunderstanding, most diseases are not introduced with new fish, even if it appears this way from observation.
The real circumstances are almost always different: There are different latent parasites in virtually every aquarium - this is normal and does not cause any harm. New fish are of course under stress, therfore have reduced disease resitance and are accordingly the first ones to be infected. The parasites then multiply strongly and then increase the infection threat to other fish, which leads to further disease outbreaks. To the aquarist, this looks as if the new fish brought a disease along with them.
Dr. Bodo Schnell