fishes for top of tank?

Hi there! I got a new male black orchid betta today and I'm super excited to have him! I'm also really nervous and maybe over-paranoid because yesterday I brought home the tiniest betta I'd ever seen, a veiltail female, but she did not make it past an hour. I believe it was completely user...

My tank is 78x40x40cm, I run an external fluval 206 filter, at the moment I have 10 cardinal tetras, 1 male cockatoo apisto (soon to be 3 as I am waiting for a pair of females) and 4 three lined corys.   When I look at my tank, what stand up the most is the vast emptiness of the top half.   With...