Finding small saltwater fish tank species for a small marine aquarium (nano tanks) is not difficult and they are often easily found in local saltwater reef stores and online. If you have a small saltwater aquarium it is vital to keep it lightly stocked and your water parameters stable. Make sure you research any fish you decide to get before you go to the store. Do not make that impulse buy or you may regret it later.
Any fish tank from 10 to 30 gallons could be considered a small saltwater fish tank for the purposes of this article. Really, any saltwater tank under 30 gallons is going to be a chore to maintain (my opinion of course, to each his own). Nano tanks are often considered even smaller. It should also be noted that the smaller the fish tank the harder it is to keep in my opinion. Larger tanks provide so many more options for both equipment and fish species and I strongly urge any new hobbyists to get the largest tanks they can accommodate. Leave the smaller specialty saltwater tank setups for when you become more experienced later on.
The saltwater fish you're looking to keep in your small saltwater aquarium need to have a few characteristics to make the list below. You also need to keep in mind how they will get along with other species along with conspecifics. For better long term success, look for fish that:
Stay small even as adults. Research the average adult size before you buy them.
Don't require much swimming space. Some fish are very active swimmers, others not so much.
Accept the more common marine fish food preparations, such as flakes, pellets, frozen, etc.
Species Suitable For A Nano Tank
Banggai Cardinalfish - these fish stay on the small side and are not very active. Feeding wild caught specimens can be somewhat challenging at first so look for captive born specimens. They are mouthbrooders too!
Pajama Cardinalfish - same as above, but most are still wild caught but they stay small and should eat most foods.
Blue Devil Damselfish - these guys are on the small side but the downside to keeping them is they can be very territorial. Keeping multiples may prove to be a problem but they are generally very hardy.
Blue Chromis - the chromis are one of my favorite species. They are awesome looking, stay small, eat most everything you can give them provided that it is small enough and they do well in groups.
Green Chromis - a great little fish that will sometimes get overlooked but they look amazing under bright aquarium lighting. Very hardy and does great in groups. They may bicker to establish a pecking order but in general they are good for smaller tanks and look fantastic in schools inside larger tanks.
Clownfish - some species of clownfish get bigger than others. They are not active swimmers, nor do they patrol a large territory. Though many folks keep them in smaller aquarium set ups, I don't think I would keep any of the clownfish species in a tank under 30 gallons as adults other than maybe the Amphiprion or Ocellaris.
Neon Goby - These are a cleaner species that stays on the small side. Many hobbyists feel they should be left on the reef but they are being captive raised nowadays...
Orchid Dottyback - these dottybacks are quite striking in coloration and could be kept in smaller tanks. They can be somewhat territorial and reclusive though and need hiding spaces.
White Belly Wrasse - sometimes picked up by reef tank keepers to help rid their tanks of unwanted coral pests. This is a good species that is generally hardy and will go after most foods put into the tank. They are farily active, so nothing smaller than 30 gallons is recommended. Lots of live rock for them to pick on is good and you also need a good fitting hood because (like most wrasses) they are really good jumpers.
Purple Firefish - I debated putting this one on the list but I'm going to anyway. I've found them to be somewhat challenging to feed at first. Once you get them eating though they are good to go. Good jumpers too so have a hood.
Yellowtail Blue Damsel - these damsels stay small but they can be very territorial. They are hardy and will eat most foods presented.
Catalina Goby - this is a cooler water species and could do well in a tank set up to meet their cooler water requirements.
There are also invertebrates such as many of the cleaner shrimps, snails and hermit crabs that can be easily kept in smaller saltwater tanks.
Obviously, the list above is just my opinion from years of being around them and don't take these recommendations as gospel. Research the species on your own and form your own opinion before you buy them. Look into the gobies, basslets, along with some of the wrasses and blennies for more ideas on small saltwater fish species.