The answer to this question depends upon your definition of a nano reef tank?
To me a nano reef tank is an aquarium which is less than 20 gallons in size.
There are some people which define a nano reef tank as being less than 55 gallons, personally I would classify these as micro reefs.
For the purpose of this post I will base the definition of a nano tank as being less than 20 gallons in size.
I am going to throw the proverbial hat into the ring by saying that under no circumstances should a fish be introduced into this type of environment.
Let me explain my reasons…
Keeping reef fish in a small amount of water is exceptionally difficult even for advanced aquarists never mind the beginner to this hobby. The majority of reef fish by their very nature are territorial which in a smaller aquarium can become problematic if not disastrous.
Introducing fish into an aquarium of this size where it is already hard to maintain excellent water conditions make it even harder to accomplish. With a fish you have to feed it which creates waste which the filtration therefore has to deal with. Even without feeding the fish are ‘breathing’ therefore adding aspects to the water for the filtration to deal with.
All of this adds up to water which is becoming harder and harder to maintain stability.
A better alternative in my opinion is rather than adding fish to a nano reef tank is to add invertebrates for example shrimps. They add motion to the aquarium and add little bio load to the filtration.
I would have to carefully consider the addition of the larger grazing animals to an aquarium of this size. By larger grazing animals I mean the likes of starfish, cucumbers and urchins. They will quickly eat all of the food and you will have to add supplementary food in order to keep them alive. Any of this food which is uneaten will break down and cause the filtration to work harder.
The addition of hermit crabs etc should be ok as long as you do not put to many into the aquarium. Hermit crabs etc are opportunistic feeders but you will need to keep an eye on them and add small amount of additional food if required.
Even without adding a fish to a nano reef tank you can have an absolutely wonderful aquarium. In some cases it can be better than having an aquarium with fish in. Without having a fish in the aquarium there is little if no predation, therefore the various pods can multiply and grow – these are fantastic to watch in your own little ecosystem plus they can be a great food source for the corals and invertebrates in the aquarium.
There are aquarists who do successfully keep a fish (sometimes more!) in a nano reef tank and have been successful in keeping them. The question that jumps out to my mind is that is it fair to keep an animal locked up in such a small space with little room to swim.
If you cannot create and maintain the perfect environment for the animals which entrust their lives to us (and this does not just include fish) then the answer is simple.
Don’t do it.