Big update!
After being disappointed with how much space was wasted in order to fit the UG filter into the round jar, and after receiving a rather large Anubias in the mail (bigger than I was expecting, anyway)...something had to change.
FredG was kind enough to point out a type of filter that would provide biological filtering with a flow rate that wouldn't bounce the betta around like a little blue pinball.
Continuing in my DIY vein (and because those kinds of nifty things aren't available at my LFS), I built my own.
Assembled and in action:
And with some plastic plants to the rescue, decoration-wise, the betta is now warming up in a little baggie
So, changes. Tank-size, obviously.
I also used crushed lava-rock rather than styrofoam in the bottom of the filter (I hadn't decided what to use yet when I drew the diagram), which should give the bacteria more space to grow and is course enough to allow water-flow without clogging the holes in the uptake tube.
I checked the PH this morning and it was reading a rather acidic 6.4, so I added in not-quite 2 cups of crushed oyster shell to the bottom of the tank. Should also keep the betta from going into PH shock since it will take some time to dissolve. Final PH should be comfortable for him.
Finally, on the recommendation of a very thorough article on Betta care, I dissolved about a Tbs. of non-iodized salt in a cup of water and added it to the tank.
The heater is keeping the water around 78-80F, and the filter is working....dunno what else to do, lol. I'll just be keeping an eye on the nitrite / nitrate levels and performing water changes as necessary until things stabilize.