The below list are suggest fish that can be kept in a nano marine aquarium. I would classify any aquarium that is less than 40 gallons as a nano aquarium. This list is only a suggested list as a starting point to help you research and pick out fish for a small marine aquarium. You would still need to verify capability and environmental/set-up considerations

10 gallon tanks

Neon Blue Goby (Elacatinus oceanops), peaceful, reef safe
Court Jester Goby (Amblygobius rainfordi) peaceful, reef safe
Clown Goby, Green (Gobiodon atrangulatus) peaceful, reef safe
Clown Goby, Yellow (Gobiodon okinawae) peaceful, reef safe
Clown Goby, Citrinis (Gobiodon citrinus) peaceful, reef safe
Clown Goby, Black (Gobiodon strangulatus) peaceful, reef safe
Greenbanded Goby (Elacatinus multifasciatum) peaceful, reef safe
Hector's Goby (Amblygobius hectori), peaceful, reef safe
Cave Transparent Goby (Fusibobius sp.), peaceful, reef safe
Two Spot Goby (Signigobius biocellatus), peaceful, reef safe
Hi Fin Red Banded Goby (Stonogobiops nematodes), peaceful, reef safe
Orange Stripe Prawn Goby (Amblyeleotris randalli), peaceful, reef safe
Wheeler's Shrimp Goby (Amblyeleotris wheeleri) peaceful, reef safe
Jaguar Goby (Astrabe sp.), peaceful, reef safe
Dracula Goby (Stonogobiops dracula), peaceful, reef safe

Red Firefish (Nemateleotris magnifica), peaceful, reef safe
Helfrichi Firefish, (Nemateleotris helfrichi), peaceful, reef safe
Purple Firefish, (Nemateleotris decora), peaceful, reef safe

Cardnial fish:
Longspine Cardinalfish (Zoramia leptacantha), peaceful, reef safe
Yellowstriped Cardinalfish (Ostorhinchus cyanosoma), peaceful, reef safe

Tanaka's Pygmy Wrasse (Wetmorella tanakai), peaceful, reef safe
White Banded Possum Wrasse (Wetmorella albofasciata), peaceful, reef safe
Yellow Banded Possum Wrasse (Wetmorella nigropinnata) Peaceful, reef safe

Yellow Stripe Clingfish (Diademichthys lineatus), peaceful, reef safe

29 gallon or 29 gallon cube
Ocellaris Clownfish, (Amphiprion ocellaris) peaceful, reef safe
Yellowhead Jawfish, (Opistognathus aurifrons), peaceful, reef safe
Spotted Cardinalfish (Sphaeramia nematoptera) peaceful, reef safe
Kaudern's Cardinal , or Banggai Cardinalfish (Pterapogon kauderni), semi aggressive, reef safe
Red Stop Light Cardinalfish (Fowleria flammea), peaceful, reef safe
Spotted Cardinalfish, or, pajama cardinalfish (Sphaeramia nematoptera), peaceful, reef safe

30 gallon (36” long tank).

Yellow Prawn Goby (Cryptocentrus cinctus), peaceful, reef safe
Sleeper Banded Goby (Amblygobius phalaena), peaceful, reef safe
Pink Spotted Watchman Goby (Cryptocentrus leptocephalus), peaceful, reef safe

Black Cap Basslet (Gramma melacara), semi aggressive, reef safe
Chalk Bass (Serranus tortugarum), semi aggressive, reef safe
Royal Gramma Basslet (Gramma loreto) peaceful, reef safe
Gold Assessor Basslet (Assessor flavissimus), peaceful, reef safe

Blue/Green Reef Chromis (Chromis viridis) peaceful, reef safe
Black Axil Chromis (Chromis atripectoralis), peaceful, reef safe
Agile Chromis (Chromis agilis), semi aggressive, reef safe

Linear Blenny (Ecsenius lineatus) peaceful, reef safe
White Blenny (Meiacanthus smithii), semi aggressive, reef safe

Orchid Dottyback - (Pseudochromis fridmani) semi aggressive, reef safe
Indigo Dottyback, (Pseudochromis fridmani x sankeyi), semi aggressive, reef safe
Neon Dottyback - Captive-Bred (Pseudochromis aldabraensis), semi aggressive, reef safe
Orangetail Dottyback (Pseudochromis coccinicauda) , semi aggressive, reef safe
Red Elongated Dottyback (Pseudochromis elongatus), semi aggressive, reef safe
Splendid Dottyback (Manonichthys splendens), semi aggressive, reef safe
Springeri Dottyback - (Pseudochromis springerii), semi aggressive, reef safe
Striped Dottyback, (Pseudochromis sankeyi), peaceful, reef safe
Sunrise Dottyback - Captive-Bred (Pseudochromis flavivertex), semi aggressive, reef safe
Yellow Dottyback (Pseudochromis fuscus), semi aggressive, possibly not reef safe
Whitetail Dottyback (Manonichthys cf. alleni), semi aggressive, reef safe
Blue Eye Royal Dottyback (Pictichromis dinar), semi aggressive, reef safe
Bicolor Dottyback (Pictichromis paccagnellae), semi aggressive, reef safe

Six Line Wrasse (Pseudocheilinus hexataenia), semi-aggressive, reef safe

Why Have I Not Mentioned Damsels ??
Damsels just might be one of the hardiest marine fish available in the hobby. Before the use of live rock and fishless cycling processes became mainstream in the hobby, a few types of damsels were commonly used to cycle marine tanks. Unfortunately, damsels can become aggressive enough to kill their tank-mates. I have seen this first hand in a friend’s tank. The reason for this is that most of the damsels will become highly territorial, and when they choose an area of the tank as theirs, all the other fish that swim by are in trouble. This can even happen with the damsels that are said to be less aggressive such as the yellow tailed damsel (Chrysiptera parasema). For those reasons, I would recommend avoiding any type of damsel in your tank unless you are experienced with them or experienced managing aggression in fish.