"starter fish"

Hi, I have a 29 gallon tank. I am not a novice at this hobby, more of
an intermediate. What fish does anyone here believe would be good to
help the tank cycle. I would like to have some discus at some point, i
dont know if that makes a difference. I was looking at some killifish.
I would be very grateful for any input!

Happy Holidays
          Evan Davis