Originally Posted by Bombalurina
I'd go with a betta, not guppies - they are active fish and will appreciate length far more than height.
Bettas are actually ok with height - I have had a longfin female in my 18 inch-tall tank and she zipped top to bottom and back again no problems. As long as you don't go for a male with ridiculously heavy finnage, like a rose-tail, I don't foresee the height of that column being problematic. Particlarly if you have some taller plants for resting spots. :)
What if I put a king plakat in, then had all sorts of tall plants like swords? Maybe a betta hammock too?