Up until yesterday morning my betta has been fine. I noticed however that it has been hanging at the bottom of the tank, slightly on it's side. He's gotten progressively worse. Before he would swim vertically, now, when he does decide to move, he darts around the tank. Here is a very crappy cellphone picture because my wife has my camera and she's out of town.

What I can see is at the base of his tail on the side pictured above he is missing his scales. Completely gone. I can see his pink skin. On the other side he seems to be fine, only a slight beginning of the same thing. I suspected he hurt himself in the plastic plants so I removed all of them from the tank.
So ... What the heck am I looking at? What's wrong with him? Body rot?
The tank has been completely untouched. It is a 5 and half gallon tank with a Tetra Whisper biofilter. I did a partial [1 gallon] water change 5 days ago and that's about it. I don't get it, what could it be and where did it show up from?
I started 1 treatment yesterday with Pimafix and I will go get some Melafix as well. Tank temp is 70, I will raise it to 78 by getting a better thermometer. Anything else I should do?
The instructions on Pimafix say I should treat for 7 days and then do a 25% water change. Should I not really be doing daily water changes?