I believe the discription you gave of the fish turning black is something many fish do who are completely stressed. The GBR in particular seems to darken in color when they are sick. This stressed condition could also be caused by housing fish together in a stressed state without enough caves, rocks, or other hiding places approperiate to each breed.
I have looked up all the fish mentioned, and they all seem to have different water conditions that are optomal for each. Some do best in 65-75F another 78, and the GBRs 76-85....one said they were super sensitive to water quality especially to chemicals that are released in the decay cycle.
I would say that you need to prune your plants, do many water changes, and even if your tanks are 5 on nitrates, it might not be clean enough.
They all sounded like they would eat similar foods, although one was a "carnivore" and required more food like bloodworms, the others could take tropical flakes, brine shrimp and bloodworms.
One article I came across which has very interesting information especially for the GBRs describes why so many of them don't thrive for us.....read this it might help.
http://www.aquarticles.com/articles/..._ramirezi.html