While ember tetras are small, they are quite an active fish and I think a group of them would do better in a minimum of 10 gallons.
It's not only the bioload you have to consider, but also the amount of space available for the fish. Schooling fish are usually a lot more active than bettas and so even the smaller species require a bigger space to move about.
Agreed with both of the previous posters. These fish may be small, but they're quite active and absolutely NEED their space!! Many profiles for fish inclide tank size, ans its usually the minimum requirement Posted via Mobile Device
I have ember tetras, and whilst they are more sedentary than other tetras, making them great tankmates for bettas, I concur that they are not ideal for a 5.5. Mine now live alone in my 16 gallon and use the whole tank. Further, in a 5.5 gallon, they don't have room to get away from the betta, and vice versa.