Tiffi: The results on this side of the pond are quite different than Germany obviously. The German blues available here are mostly imports from Asia. There is documentation of what the breeder's (fish farms) there are doing to maximize spawns, as well as coloration. This has weakened not only the fish itself, but has weakened the gene pool.
This has resulted in a fish that is "sensitive" to not only water conditions, but acclimation, diet and just about everything else that can/could go wrong. It is not uncommon for a hobbyist to purchase a GBR and have it die prematurely.
The research I did makes it clear that you can be successful if you have fish that were spawns of a wild caught...up to F5. Or, healthy spawns of dedicated breeder's.