The Nano Reef Tank – Should You Keep Fish In Them

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.

  1. Wow, just wow. This author has done a poor job researching actual nano reefs to make such sweeping generalizations.

    The facts are that some reef fish naturally live in very small territories or are very small themselves. For example, ocellaris/percula clownfish, clown and most other gobies (such as shrimp gobies), and small wrasse like possums or Pseudocheilinus are all suitable for small tanks. For non-reef specimens, dwarf seahorses are another option, but they are legitimately demanding.

    It is NOT hard to keep a nano (or “pico”) reef stable once you understand the rules. The only major differences between them and larger tanks are resistance to environmental temperature swings, salinity increases from evaporation, and proportionately smaller bioload capacity. In fact, many nano-reefers keep SPS corals and tridacnid clams, which require stricter water parameters than most fish.

    Nano reefs are not death traps for all fish. It is true that the majority of fish offered by LFSs or online suppliers are too big or active for a nano-reef. Instead these tanks allow one to keep fish, like a Trimma goby, that would be lost in a large tank. They also allow species-specific tanks for fish like Pseudochromis.

    For those poor souls who are throughly discouraged by this article, I suggest going to the website I’ve linked in my comment. is one of the most helpful forums for keeping nano reefs, and nano fish, healthy and active.

  2. Hi lakshwadeep, and thanks very much for your comments.

    Those that are targeted by this site are beginners. This is often missed by many and, as the days go by, is sometimes not made entirely clear by authors. Reading your text I would gues that you have experience, noting your awareness of fish types and the general language you use.

    It may be a surprise but I do not disagree with you, but at the same time I know what Peter is getting at.

    There are two major problems (there could of course be more) that beginners make, even if they are under the wing of their LFS. The first is overstocking, and the second is failing to maintain high seawater quality. Both are definitely undesirable.

    Seawater quality suffers because overfeeding can easily occur and is arguably the number one failing of a new aquarist. It occurs for the right reasons too, that is the aquarist is concerned that his/her charges get sufficient to eat and at the same time is enjoying the pleasure of feeding. Over heavy feeding can reduce seawater quality quite rapidly. Routine seawater changes are not always entirely understood by new aquarists and these could be missed or insufficient. If overstocking has also occured then the problem is exacerbated.

    Also sometimes a new aquarist has been somewhat taken aback by the cost of a larger marine system and decides to buy a smaller one. Perhaps there are memories of a goldfish tank years ago, or a tropical fish aquarium that was small and manageable. This can cause a small marine system to be attempted, in other words it could be a money saving adventure. This does not always bode well for the success of the venture.

    I have been quite surprised sometimes by the basic lack of knowledge and confusion that is demonstrated by some as indicated by questions that have been put. This does not mean that the persons are in any way ‘lesser’, it is simply what it is, a lack of understanding of basic knowledge possibly caused by confusion.

    We try to meet the needs mainly of beginners by putting matters in a straightforward way. Peter has, reading between the lines, tried to help take the problem away by advising ‘don’t do it.’ As said the target is beginners, not experienced aquarists. We tend to talk of ‘guidelines’, not rules, and in this case I guess that it was a strong advisory not to stock fish with very small net seawater gallonages until the basic knowledge needed for maintenance is understood. This means some research and that, sadly, is what is so often missing.

    As already said earlier, I agree that fish can successfully be kept in small aquariums. However, success is often going to be a long way off if the beginner doesn’t carry out the necessities of maintenance.

    Once again, thanks for your comment which I will bring to Peter’s attention.

    Johns last blog post..Just Relax!

  3. As John says thanks for the comment.

    There was never any intention to say that fish should never be kept in a nano aquarium. It is just my personal opinion that beginners to this hobby should gain experience first in providing stability before considering adding fish to a nano.

    Fish, as you know present extra load on the aquarium filtration and the inexperienced aquarist may find it difficult to maintain stability until the aquarium has settled and they have gained experience both through research and through the practical hands on approach. In my opinion new aquarists need to learn how to provide stability via performing the essential care and maintenance and learning how to look after the livestock in there care ensuring that they are not overfed etc.

    There was also no intention whatsoever of putting people off starting a nano aquarium via this post. As you say is an excellent resource and I would recommend anyone interested in starting a nano aquarium to visit it.

  4. Sorry, my tone was too harsh. I agree that stability is prime, and puts a great stress on weekly water changes (20% of a 10 gal tank is much easier to exchange than a 100 gal), enough live rock, and limited stocking levels. I started my nano reef adventure in 2003, so I probably have not remembered my mistakes and assumptions. I have often kept a fishless tank for long periods of time, so I understand the relative increase in stability of such a tank.

    I agree that it can be shocking what beginners will come up with when they ask a question. Territoriality is a good issue to discuss. Overfeeding is bad, but since nuisance algae, to the horror of the owner but to the benefit of the tank, will quickly use up excess nutrients, I don’t know if that would be so detrimental. I also admit that I’ve never kept dedicated freshwater setup, so that may have spared me from bad habits like dumping fish food. I agree that heavy overfeeding can be disastrous.

    I think the basic cause of me choosing to argue was to call out the implication that a fish in a nano is equivalent to being “locked up”. In the wild, clownfish stay in their anemones, clown gobies live in SPS colonies, and shrimp gobies won’t venture far from their burrows. Some tanks may still be smaller than what is normal for a fish to use, but I don’t see how this is any different than many of the fish that are constantly kept in larger tanks. Things like tangs or even the apparently diminutive dwarf angels have extremely larger territories (up to thousands of square meters) in the wild, and it can be short-sighted to say that these fish are okay to be kept in larger tanks when a clownfish or goby is off limits for a nano.

  5. Don’t apologise, constructive criticism is very welcome!

    Yes, you’re right again, comparing the larger territories of fish kept in a large tank to the tiny needs of fish that should be kept in a nano appears to destroy the argument re fish that should be kept in a nano.

    I can only re-iterate – what fish do beginners often choose, will it be the correct fish? It could be a clownfish, but it is not unknown for a tang to find its way in! Horror, didn’t the LFS ask any questions?

    Johns last blog post..The Banggai Cardinal At Risk?

  6. I think this article could have been written with far less conviction:

    “Under no circumstances should a fish be introduced into this type of environment.”

    That comment just shocked me, coming from somebody who at first seemed to know what they were talking about. Sure, we don’t want to encourage people to throw unsuited fish into a nano, but that doesn’t mean you have to lie about things. There’s plenty of fish suited nanos, just look at the symbiotic relationship between the pistol shrimp and the blackray goby for example – both beautiful fish that would be lost in a large reef tank, and perfectly suited to a nano with sand and live rock.

    With small and few fish, water quality isn’t that difficult to maintain. Sure, you have to do things differently – more frequent water changes, less room for error, all you had to do was reiterate the key points for research and where things can go wrong with a nano, I just don’t understand why you took that approach to the article, it’s like a car salesman refusing to sell sports cars because some people might have abig family. Strange analogy, maybe, but please don’t mislead people, there’s enough misinformation on the internet as it is!

    A disgruntled (well researched) beginner.

  7. Yep!. We’ve already accepted that the article is too strongly worded, see the earlier replies by Peter the author and myself.

    We don’t ‘lie’ though! We strongly object to that! We do have strong opinions however and take the side of the often unfortunate fish and corals that can so often end up in an unsuitable environment.

    I note that you describe yourself as a ‘well researched’ beginner’. Well, that’s good without doubt. Research is so necessary but research is often the item that isn’t done. This is the area sometimes causing loss of unfortunate fish and corals. When lost the aquarist sometimes even just replaces them believing that it was simply bad luck. Eventually, experience and probably expense start to bite and questions are asked. If only those questions where asked to begin with.

    So, to re-iterate, yes fish can be successfully kept in a nano as long as suitable fish are selected in the first place. This selection is down to the purchaser as only too often a retailer will simply sell a fish on request. Also seawater quality has to be up to standard and again a beginner will often fail to realise the full significance of this.

  8. Hi,

    Nano reef or not nano reef. Its not ok to put any animal in any kind of captivity. I have a nano-reef myself. with sps, lps and FISH!:)

    bye, Marko

  9. Hi Marko.

    I have to admit I don’t understand the comment. You state that it isn’t ok to keep any kind of animal in captivity – but then say you keep fish in a nano reef?

  10. I was considering buying a nano tank for my fish, I didn’t know that it wasn’t a good idea! Thanks for the heads up!

  11. Hello. Please read the comments after the article. A nano aquarium can be successfully used for fish – as long as the fish are carefully selected as suitable for the size of aquarium. This applies to any aquarium of course, but with a nano it’s more critical.

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