Nano Reef specs?

[Tadashi]

Is there an accepted definition of nano reefs? Below a certain size? No sump? ...

Five years ago it used to be anything that was under 15-gal.

It seems that Pico reefs are still 5-gal and smaller.

[iFisch]

There's no general rule. There's always exceptions. And it depends on the dimensions of the tank.

Usually ~30g is "nano". Since the 28/29g AIO's have become SO popular.