The smallest functional tank I have seen is called a picotank, and it was a dual betta hex tank. This has a divider in the middle, and one side was the refugium. I have also seen a bettavase tank. Obviously, it is highly specialized and requires a lot of attention. No doubt, however, that with smaller and smaller PCs becoming available, that people will go crazy. But I am assuming you are actually looking for practical advice
So:
I have an 8g and a 15 g. Both are nice sizes to work with. The minibow7 is a popular tank, as are many of the eclipse systems although most will need lighting retrofits. I don't do much more than a few polyps in mine because I am more interested in my brittlestars. Getting down below, hmmm, maybe the eclipse 6 really starts to get, IMO, into an even more specialized area than just nano tanks...the 2s and 3s, IMO, are getting tricky.
The general problem with the eclipse tops, apart from lighting, is trying to modify them for something like a small skimmer, if you choose to go for one and have the top for the 10g and up. Water changes each week, however, may make this uneccessary. The other factor is the lighting retrofits. Once you get further into it, you may check out in DIY and see if they have done any modifications or anything.
I generally encourage you not to do more than 1 fish in the nano/10g size range, although a pair of small clowns is doable. Obviously, the more LR, the better, but there is some evidence that you may not wish to waste the space on something like a DSB. There just may not be enough surface area for it to be extremely functional. I learned this late, but then, I like little wormy things anyway.
You can probably do some searches for these different size tanks...or perhaps even find some mantis keepers since they tend to keep these guys in smaller eclipse style tanks.
In short, there are a lot of options in the smaller tank range. I wouldn't always recommend this, if you were just starting out in the hobby...they do require a lot of attention, and fluctuations can be pretty dramatic sometimes.
If you have any other questions, you can write me at
ophiuroid@att.net