Scaly Face Mange Mites are a possibility that perhaps came from the bird's parents and were hidden until now. Mites are not common infections on well kept, indoor birds, but yours are still pretty young and may have been exposed before coming to you.
There will a powdery look to the scaly part of the bird’s feet and skin. As the infection takes hold, upon close examination the area will look like a honeycomb and the growths can become so big that they hang from the feet, thus another name for this is “tassle foot” .
To diagnose these mites, a skin scraping is required by a vet.
Knemidokoptes or Scaly Face mites are often behind a condition called “bumble foot”, but can become a lifelong problem affecting the beak, cere and face.
A bird with knemidokoptes will usually show discolored cere, growths or projections near and on the nares/nostrils and depending on far along it is, peeling, crusting, powdery flaking or deformation of the beak.
There are no home treatments for this very serious condition and quick professional treatment is required in order to have the best chance of full and uncomplicated recovery.
Ivermectin specifically for birds is many times used to treat, but it’s only through vets and must be formulated for your individual.
There is more info here:
http://www.avianweb.com/scalylegs.html
Another problem that may arise in multi-housed birds is over-grooming each other (or one may over-groom the other). You may end up with a very shredded looking bird that’s being feather plucked by a mate (they can be two males or two females as well). When this happens you need to be sure it’s not being caused by an underlying parasite that the plucking bird is trying to resolve on their partner so a vet visit is always a good idea.
You should also double check the size of their enclosure and perhaps provide something even slightly larger.
Adding a food and water dish to the ones already there might also help them resolve this if it’s territorial aggression. Even if they continue to share the same dishes, at least they know the option is there.
Changing out some toys, bells, other interactive features may give them something different to focus on.
---- Finally, make sure the living arrangment is large enough for this number of birds.
Even though they're free flying during the day, they need their own space to roost at night, especially if you have mixed sexes and they're at maturity (1 year or older).
For 8 birds you'd need at least a 4' tall x 3'wide/deep sleep cage.
It sounds like you really love your little companions. You're doing a very good job with their sleep hours. You can even increase it to 12 if you'd like a more natural balance for them.
If they're not exposed to natural, unfiltered sunlight during the day, try installing a full spectrum light made specifically for birds (not the ones for reptiles or plants) - this will also help prevent any feather plucking and some other problems from happening.
Keep up the really good care!
Certified Avian Specialist
Cert. Avian Specialist; Int. Assoc.Animal Behavior Consult; Pet Ind. Joint Advisory Council; author