Hi, Doug! <waving>
Urkevitz, I've run the same tank with no filtration. It's so tiny, it's not really necessary. Then again, I had a 25W Hagen Thermal Compact heater so I imagine there was usually some heat convection helping to move the water a bit. I kept a few guppies, too. What I did for CO2 was to put an over-turned bottle cap on the "exit end" of the tubing to act as a rudimentary bell diffuser. It was more than enough to keep the CO2 levels sufficiently high, but a lot of this is going to depend on KH. One time I got very careless and the pH plummeted to way lower than was safe (especially for guppies). First thing that clued me in was the disappearance of snails. The lack of current really helps to keep the CO2 in the water.
Later, before the AZOO palm filters ever came out, I put an AquaClear MINI on the tiny tank. I liked having a filter, but I had to do a LOT to slow down the flow so that it wasn't a monsoon. I think I eventually took it off again.
I do know of somebody who has the same set-up as yours, does NOT inject CO2, fertilize, filter, or heat his tank, and grows a ton of erect moss beautifully. He set up this tank specifically for keeping and breeding shrimp. He e-mailed me a photo of the set-up and it's beautiful.
Anyway, I found it unnecessary to have filtration, but now that they have these AZOO/RedSea palm filters, I just go ahead and use them. They don't hurt, and the fish seem to prefer a little current.