Ok everyone seems to have taken tangents with heaters and things so i'll give you the stocklist of my tank. I have a 20g high tank that i am in the process of stocking with 2 schools of 9 and 6 Kuhli loaches. 9 Neon Tetras are one school and the other school is Harlequin Rasboras and i still need 5 more to make up the 9 of those. I was only able to get the 1 Kuhli until they restock at the LFS as well. The Tetras and Rasboras get along well and they follow the Kuhli when it comes out like it is the parade leader. I could have gone for 1 school of either NT's or a school of the HR's and a pair of Blue Rams instead of the second school while still keeping 6 Kuhli Loaches.
So maybe think about
1 school of about 8-9 Tetras or Rasboras
1 school of bottom feeders like a small species of Corydora (possibly about 5-6) or Kuhli loaches (who might only come out while you sleep)
1 slightly larger fish that will really catch the eye. Seen people suggesting dwarf gouramis although i know nothing of them. I was going to have 2 Blue Rams
Or you could use something like my stocklist.
9 Neon Tetras
9 Harlequin Rasboras
6 Kuhli Loaches
When i am fully stocked i will think about adding a few cherry shrimp. My tank is very heavily planted and have 30g filtration on a 20g tank so i will go over the recommended stocking by about 6%.
You can use
aquadvisor.com To run the numbers you can keep when you decide what you like.