Deep is a varied term and what’s considered somewhat / moderately deepwater might not be much to one of those Asian collectors with deep connections and deep pockets, or someone like Andrew Sandler. If we define deep as “more / most common below 100 feet”, I can think of a few:
-Black Cap Basslet (G. Melancara)
-Candy Basslet (L. Carambi)
-Helfrichi Firefish (N. Helfrichi)
-Tinker’s / Declivis / Mitratus / Burgessi Butterflyfish (C. “Roaps” Tinkeri / Declivis / Mitratus / Burgessi)
-Many Genicanthus angels (Spot Breast, Watanabei, Bellus, Japanese Half-Masked)
-“Maybe” (not sure about this one): Gold Flake Angel (A. Xanthopunctata), Griffis Angel (A. Griffisi), Bandit Angel (A. Arcuatus)
-Many Anthias (I don’t know all the specific species but do note some are difficult), a common deepwater one different from your typical Anthias is the Blotchy or Borbonius, also the Sunburst Anthias is different from your typical Anthias
-Starck’s Damselfish (C. Starcki)
-Dwarf angels (Colin’s, Multi-Bar, Venusta- none of these are easy)
-Some wrasses (not sure about specific species)
-Prognathodes butterflyfish
TBH I love cool deepwater fish (including the really deepwater ones found most commonly below 300 ft but those tend to be $$$$ or totally unobtainable)
One thing to keep in mind when going for a deepwater tank is many species suffer from decompression issues if not handled well.