[QUOTE=Looks terrible, there's that sore-looking spot, he is pale like I've never seen, and his tail looks torn up...he's currently swimming around lethargically, the other guppy is hanging around him like he always does, but not harrassing him or anything

Any ideas what the heck that was/is?[/QUOTE]

Just got finished treating my tank for the same thing. Lost 5 guppies and an old cardinal tetra over 5 days before I caught on it was not fin nipping induced. Did a ton of reading. Came to the conclusion it was a bacterial and fungal infection. I treated with Furan 2 and paraguard for seven days and a ton of tank cleaning, water changes and increased circulation. I learned "fin rot" can be a symptom, not a disease. Look into Saprolegnia if the problem persists.

Hope I could help. :-)