These are the questions you will always come across when reading a nano reef forum. I do not believe their is a definite answer to this, I do however personally feel that anything over 20 gallons is not a nano and 15 gallons is pushing it.
In my opinion, a true nano, in every sense of the word is up to 10-12 gallons (not including a sump).
Can a nano reef be as successful as larger tanks?
Yes, even more so. The most important factor in keeping any reef aquarium is water quality. You can have the fanciest lights, the “best” additives…ect, if your water quality is not spot on, you will not have a healthy reef. The easiest way to maintain optimal water quality is by performing water changes. Since nano’s have such low water volume, one can perform 50% water changes weekly without even pulling a sweat!
All you would need to maintain your tank is salt and an RO-DI unit! Yes, you still need the obvious items such as lighting, temp control, water movement, food, etc, but you will most likely not need any type of filtration. No protein skimmer, no carbon reactor, no phosphate reactor and no mechanical filtration either. You will most probably not even need to worry about dosing for calcium, alkalinity nor magnesium!
But why, how?
Easy, you will get EVERYTHING you need from a simple water change! Remember, you have a 10 gallon tank. Even if you mix a 5 gallon bucket of fresh salt mix weekly, you will be changing out roughly 50% of your water. A 50% water change will most defiantly replenish any calcium, alkalinity and magnesium and it will also remove all the “bad stuff” that a skimmer or sump would have removed!
The only issue I have with large 50%+ water changes is they could drastically alter your water chemistry. This would be ok for a fish only tank, but if you have any type of coral, I would NOT suggest it. What you could do is mix your 5 gallon bucket of fresh salt once per week BUT only change a couple of gallons at a time, say every couple of days. This way you still get the benefit of a 50% water change weekly without shocking your corals with any drastic changes.
Important note** Keep the mixed saltwater covered to protect it from stuff getting in and to minimize evaporation. If it evaporates, your salt mix will be comprised. As you are probably already aware, salt does not evaporate, only water does.