I was thinking "nano" in the sense of the teacup size hells I see selling everywhere.
After my goldfish, etc, I dont have room for 3 15 gallon tanks.... I dont know that I have ever SEEN a 15 gallon, Ive seen 10 and 20 (and larger), and maybe a hexagonal, Jules Vern-ish, steampunky 12 gallons.
But no 15.
So far as decor, I do have 2 psychdelic crazy looking aquariums (for my kids amusement, and the fish/invertabrates are very healthy) but my fancies and bettas are definatly more reserved naturalistic decorated tanks. But I don't see what you are driving at so far as decor. That's a personal decision, so long as the decor is meant for an aquarium and is functional and safe for the fish.