If you do decide on a new (different) filter for this 75 liter/20 gallon tank, I would suggest a simple dual sponge connected to an air pump. You have fish that are sedate (so far those mentioned are, which is best in smallish tanks anyway) and this means less rather than more water current. I use sponge filters in my smaller tanks (from 10g up to 40g). Larger tanks seem better with external canisters.
To the mentioned fish...the GH is soft to moderately hard (160ppm = 9 dGH) so this is easy to work with, especially with a pH slightly acidic (just below 7.0 which will likely be the case once the tank is biologically established). The neons are fine as is. Ember Tetra would work, in a group of at least 9-10. Small-size fish always do better in larger groups than larger-sized fish, with some exceptions, but generally, with small species, keep the groups around 10+. This also applies to the Corydoras habrosus (or the other "dwarf" species, C. pygmaeus and C. hastatus). A group of 9-12 of any one of these species would be fine. Make sure you have a sand substrate however, these do not fare well over gravel.
Another parameter with the species so far considered is temperature. Neons, the dwarf cory species, and Embers are best around 75-76F/23-25C but no warmer. So if any other fish are considered, keep this in mind. Temperature is even more critical for small-sized fish because it directly drives their metabolism.
A 20g/75l is small space to fish, so it is best with smaller species because not only will water quality be more stable but you can have more fish in the space, which adds interest which is important for the aquarist. My fish room is full of tanks of smallish fish, nothing much over 2-3 inches with a couple exceptions, and this can make for a very interesting aquarium.
Byron.
