I consult these boards fairly regularly but until now have never needed to ask for help, but I think the situation needs it.....
Last week I lost my poor little dilute gouldian boy. I've only had him for two years and he was the first Gould that I actually bought from a bird store, so he was kind of special. he looked poorly when I got him, but I just had to take him home

Over the last year and a half he really went downhill though. First I suspected worms, so I worked him and the rest of the multicoloured flock. He didn't change. Then I treated him for air sac mites at the vet's direction. No change still. Someone then suggested adding some extra greens to his diet. He was't overly keen on sprouted seed but loved kale and spinach. He perked up slightly. Someone then suggested I pop some diatomaceous earth into the seed and sprinkle it around the aviary just in case it was a non-blood sucker that was attacking him. He remained perky but still unlike the other Goulds.
Meanwhile the canaries, owlies, painteds and cordons were looking fantastic! Never seen them happier!
I have no idea what did him in. I really don't. Our weather has been mild, if there have been storms I have moved him inside, and he hadn't bred. His feather condition looked pretty bad, but nothing I did helped him. The body had no signs of trauma (no blood or broken wings) and none of the others are looking like they are ill.
Was it something I have done or could it just be that he was a dud from the start? Is it something with the dilutes? The bird shop said he was about 8monrhs old when I got him. He was also wormed in front of me and mite treated as well. As a precaution I've dusted, sprayed and I ivermectin treated the remaining flock..... I don't understand why he died

Has anyone encountered this before?