A 10 might be ok. I had them in a 10X 10" cube. I do know that most hobbyists that keep them, have them in tanks as small as mine and smaller.
To me, size alone does not constitute a good candidate for a nano. Whether it is a sw or fw fish.
How big will the fish get is a question to ask, of course. The most important thing is swimming pattern/habit. If a fish is very active, it needs room and a higher oxygen content. Some hobbyists who keep nano reefs think it's ok to put baby fish such as small blue tangs in nanos. They counter 'I will take him out when he outgrows the tank'. Some fish such as tangs require a much higher oxygen content than most nano tanks afford. Not to mention it's cruel to keep a fish that has the potential to grow to 16", in less than a foot of space for any length of time beyond holding at an lfs. I don't think bettas should be in bowls without filtration & a heater either.
I tried three yellow Kubotai rasbora in my ten gallon. I had to give them to a friend with a bigger tank. They were darting and pacing the tank constantly. It disrupted my axelrodi rasbora, which are more mellow. So, that's my take on it. I really wanted the killifish. I joined forums and researched for over a year on it. Most people seemed to concentrate on how pretty they were for a fw fish. Photography is big with them. They do not get into the asthetics of setting up a nice looking tank. It's mostly about breeding. I am not in this hobby to put water in every vessel in my home that will hold it.
{to spite what my sig. says..
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