Not sure about the Eel, but not an expert. I’ll try to go over the others but please don’t take my word as gospel, I’m someone who wishes he had a tank already but stuck abroad due to Covid.
1. Fox Face- Good algae eater, hardy, not too bad in temperament. Any variety would fit, but the Bicolors and Magnificent are the prettiest.
2. Naso Tang- Not sure, they really need an 8-10’ tank IMO. They’re very much open water swimmers, pelagic I think, rather than more reef associated like other Tangs. They can also be not the hardiest fish.
3. Blue Tang- Maybe, but please check as these are also fairly large and active. They can be a little more prone to ich but seem to be hardy once established. I think some places in Indonesia might be breeding them in captivity but not sure about availability in the States.
4. Copperband Butterfly- Be sure to have a variety of foods ready, try to get an Australian specimen if possible though that might not always be easy. Start small, establish before more aggressive fish. Maybe check to see if it’s eating at the store. Fresh clams, shrimp, and chopped seafood would be a good start.
5. Tomini Tang- Great. Would make a dynamic algae eating duo with the Foxface.
6. Achilles Tang- These are some of the most active and least hardy Tangs out there, if you can find one. They live in very high flow areas with brisk water movement. They practically require a solid QT / prophylactic routine more so than the average fish or Tang does. Not sure about their size in captivity but don’t think they’re the largest Acanthurus.
7. Clownfish- I’m not a fan of the designer Clowns. If you want something peaceful but bored by the standard Ocellaris or Percula or Darwin Clowns, maybe give the Orange and Pink Skunks a try as they’re captive bred too.
8. Royal Gramma- Great, small and full of color. If you add them at the same time you can maybe do a harem like 3.
9. Yellow Coris Wrasse- Great color, personality and pest eater.
10. Six Line Wrasse- People have mixed reviews of them when it comes to aggression. Some say they’re devils among wrasses but others do just fine with them. If you have a big enough tank like your 900, maybe you can add one but as the last wrasse.
11. Leopard Wrasse- Can be difficult to get eating, this is another fish I’d have a variety of foods for upon introduction. Meleagris I heard is a relatively easy one for the genus.
12. Firefish- They need a confirmed pair unlike their Dartfish cousins, but a 900 should have enough space.
13. Aiptasia Eating Filefish- Get CB from ORA or Biota if possible. Interesting, some people say they eat SPS and softies.
14. Ruby Red Fairy Wrasse- Great.
15. Adorned Wrasse- Great.
16. Radiant Wrasse- Some say they don’t ship as well as other Halichoeres and seem to just die for no apparent reason, others say they’re just as hardy as other Halichoeres. I’d ask.
17. Possum Wrasse- Can be very secretive and tough to eat eating at first. They’re pod hunters that want some pods in their diet though not to the same degree as Mandarins. You might not see yours very often in a 900.
I would highly suggest a smaller Zebrasoma Tang in place of the Naso (Yellow, Purple, Gem) and a different species of Acanthurus (Mimic, Lavender, Convict- if you can find a healthy one that’s eating well, maybe Gold Rim).
You should also consider dwarf angels, some bottom dwellers, more peaceful damsels, and some other relatively reef safe butterflies (though expensive) like the Roaps subgenus or maybe the Lemons if you can find them captive bred from Biota. Pyramids and Zosters are plankton eaters and relatively reef safe.
Some of the Genicanthus angels are good too. You can mix females or do a pair. Captive bred Singapore and Blue Line angels from Poma Labs, Majestic Angels, Goldflake Angels, Regal Angels would all be options for larger angels.
Goldflake, Majestic can be found captive bred. Japanese Swallowtails are captive bred by Biota though not sure about their availability. Start with 2 smaller females and hopefully 1 will change to a male over time.
