OH MY GOODNESS I AM SO STUPID IT'S FINROT! oops sorry for shouting. I feel so dumb! It all makes perfect sense.
See, a while back my Otos had finrot. I didn't have a q tank but I treated them successfully, and since I figured the Guppies and Platys were pretty much invincible I never gave it another thought. Well, two of my Guppies caught it--Corona (the "dying" one) and Sunspot--when I missed a water change and the nitrates must have built up higher than their liking. Corona, being a slightly snobby little girl, was apalled to see her fins in such a condition and sat on the bottom and sulked. Of course--and this is what got me--she never clamped her fins or anything, because she really is not that sick. But, since she sat on the bottom and wouldn't even come up to eat, I couldn't really see her fins and I never reallly thought to look closely before. It looked like she was dying, the way she just sat there. Sunspot was more active but clamped her fins a bit, and wouldn't eat either.
So...another round of finrot treatment. Fortunately it doesn't seem as advanced as the Otos' cases. The Guppies just seem much more unhappy about it than the Otos were. Good grief, I'm going to be an expert at finrot treating by the time this is all over. I think I'm going to try to get them a bigger tank. The nitrates in my 10 gallon just build up too quickly, and then I end up with finrot.

At least it isn't some life-threatening disease. The little stinkers are just big sulkers. :

Goodness, that probably took 5 years off my life.

Whoopee, another round of finrot, and just before my Grandma arrives from Arizona too!

--NOT