I don't know how it relates to aquaria, and I may be telling you something you already know, but micro, nano and pico are genuine terms of measurement.

If you start from zero and go up by a power of 3 you get thousand, million, and billion respectively

If you start from zero and go down by a power of 3 you get milli, micro, nano, pico respectively.

eg...

if you start with 1watt, and go up by a power of 3 (i.e. you add 3 zeros), you get 1000watts = 1kw.

if you add another power of 3, you get = 1,000,000 = 1MegaWatt

going smaller....

1watt, down by a power of 3 = 0.001w = 1milliwatt

and again by a power of 3 = 1microwatt

You can use it to measure lots of things my example is for power, but it could be weight, size\distance etc for example.

So in the true sense of the phrase micro - a micro tank would measure between one thousandth and 999 thousandths of a millimeter!

I hope that makes sense and is insightful!

I'd be interested to know if there is another 'genuine definition' with regards to auqariums. I suspect not though, I suspect that someone just took the terms and misapplied them to aquaria as they describe something small. ...