I've got a 24gl Aquapod which I believe is simular. I put my heater in the pump chamber and it works well. Currently, I am using Matrix, Purigen and Phosguard. I am also using a polyfilter. I am sure you will hear various comments on what you should use and by no means am I saying these would 100% correct. Some use activated carbon, bio balls, standard filter sponge, live rock rubble, etc. It can be a bit confusing at first but you will figure it out with some help.
Good luck.