Any marine tank under 40 gallons is inherently unstable. As the saying goes, 'the solution to pollution is dilution'. Nano reefs contain minimal water volume and as such, management requires diligence, consistency, attention to detail and wise choices when it comes to stocking your tank. You can be the best water keeper in the world, yet never be able to keep up with the half-eaten snails attacked by a longnose hawkfish.
Reading and research is the best way to go. If you decide that there is ONE particular animal that you would like to have in your tank, research it thoroughly. Know it's compatibility with other fish, it's eating requirements, how large it grows, aggressiveness, etc. Build the tank around it if you MUST have it. Otherwise, plan carefully as you go. Here are a few nano tips to keep in mind when stocking your tank:
1) Avoid fish that will not stay small. Avoid fish that are going to reach 5" or more unless it is the ONLY fish you are going to keep in the tank.
2) Peaceful fish go in first, aggressive go in last
3) Keep in mind what it needs to feel biologically safe (caves, deep substrate, a symbiotic co-inhabitant)
4) Avoid the 'just one more fish' urge
5) Stick with hardy animals that can survive with the sparse feeding that goes with a nano; avoid fish that require contaminating foods (mussels, squid, fish)
6) Avoid same species pairs except for paired mates, schooling fish and other situations (paired percs, eg)
Below are some stocking choices that require due diligence and perhaps, let's say, you should be cautious about. Some of these animals have caused problems in a nano reef situation. There are anecdotal success stories and some people may have a species tank with them, but, in general, 'consensus' is generally negative in a nano tank. Feel free to comment or add.
Inverts
1) Nudibranchs, sea cucumbers
2) Sea stars
3) Sea apples
4) Anemones
5) Coral banded shrimp or boxer shrimp
6) Sally Lightfoots
7) Flame scallops
8 ) Arrow crabs
Fish
1) Hawkfish
2) Non reefsafe wrasses
3) Various dottybacks
4) Fish that grow over 5" (tangs)
5) Flame angels
6) Any poisonous fish if you have kids in the house (some blennies, eg)
Corals
1) Galaxea
2) Goniopora
3) Non-photosynthetic gorgonians
4) Some corals can act like weeds and overrun your tank. Some include pulsing xenia, yellow colony polyps, green star polyps (GSP)...although they don't pose any direct caution per se.
Macroalgae
1) Some macroalgaes if placed in the main tank and under the right conditons, can take fast on the LR and overrun it.
2) Caution with caulerpa...this macro can go sexual and release spores and stored nutrients into your tank.
This is only meant as a springboard for research. It may pay to avoid many of the above during your initial startup phase, ie, take a conservative approach and graduate up to more difficult additons. Hope this helps. SH
Reading and research is the best way to go. If you decide that there is ONE particular animal that you would like to have in your tank, research it thoroughly. Know it's compatibility with other fish, it's eating requirements, how large it grows, aggressiveness, etc. Build the tank around it if you MUST have it. Otherwise, plan carefully as you go. Here are a few nano tips to keep in mind when stocking your tank:
1) Avoid fish that will not stay small. Avoid fish that are going to reach 5" or more unless it is the ONLY fish you are going to keep in the tank.
2) Peaceful fish go in first, aggressive go in last
3) Keep in mind what it needs to feel biologically safe (caves, deep substrate, a symbiotic co-inhabitant)
4) Avoid the 'just one more fish' urge
5) Stick with hardy animals that can survive with the sparse feeding that goes with a nano; avoid fish that require contaminating foods (mussels, squid, fish)
6) Avoid same species pairs except for paired mates, schooling fish and other situations (paired percs, eg)
Below are some stocking choices that require due diligence and perhaps, let's say, you should be cautious about. Some of these animals have caused problems in a nano reef situation. There are anecdotal success stories and some people may have a species tank with them, but, in general, 'consensus' is generally negative in a nano tank. Feel free to comment or add.
Inverts
1) Nudibranchs, sea cucumbers
2) Sea stars
3) Sea apples
4) Anemones
5) Coral banded shrimp or boxer shrimp
6) Sally Lightfoots
7) Flame scallops
8 ) Arrow crabs
Fish
1) Hawkfish
2) Non reefsafe wrasses
3) Various dottybacks
4) Fish that grow over 5" (tangs)
5) Flame angels
6) Any poisonous fish if you have kids in the house (some blennies, eg)
Corals
1) Galaxea
2) Goniopora
3) Non-photosynthetic gorgonians
4) Some corals can act like weeds and overrun your tank. Some include pulsing xenia, yellow colony polyps, green star polyps (GSP)...although they don't pose any direct caution per se.
Macroalgae
1) Some macroalgaes if placed in the main tank and under the right conditons, can take fast on the LR and overrun it.
2) Caution with caulerpa...this macro can go sexual and release spores and stored nutrients into your tank.
This is only meant as a springboard for research. It may pay to avoid many of the above during your initial startup phase, ie, take a conservative approach and graduate up to more difficult additons. Hope this helps. SH
I think you should include sun corals in that list lol. Sure are pretty but a real pain in the butt to keep you prams in check. My nitrates gone from 0 to 10 since i have had it lol 
