Of all the concerns to be brought up about keeping a nano I’m surprised the cost of an ato is an issue. In the grand scheme of saltwater keeping I don’t see an ato as expensive. I use an autoaqua smart nano which is about $135. It’s an important and clever bit of kit and I don’t begrudge the price at all. Even on a nano you
can spend many hundreds of $ on lighting so what’s a bit more on an ato.
I think there might be a misconception that people who want a nano tank want to do it as cheap as possible but you’ve only got to look through this forum to see that’s not true. I think most people keep them like any size tank, things are just scaled down - fewer lights, smaller pumps etc which in turn happens to make it cheaper to set up than larger tanks.
I went for a nano because of space limitations. I could upgrade from my Fluval Evo 13.5 but with the space I have available it would still be classed as a nano.