Molly looks terrible.

My molly has been swimming with his head up for the last few days, about the same time I found a large black spot on his stomach. It looks like an internal problem rather than an external one. He's been flapping his fins and remaining in the same spot in the aquarium rather than swimming around like he used to. I've also found him resting at the bottom of the tank, barely moving. My dalmation molly has been chasing him around, whereas he used to be the one who did the chasing. It scares the heck out of me, as he's one of my first attempts at fishkeeping and I'm very attached. ):

I did a 25% water change today and I'll continue keeping an eye on him. Any insight as to what's wrong with him would be appreciated.
I don't have a quarantine tank cycled, so my options are to leave him in or euthanize him. I don't want him to infect the others if it's contagious.

He's in a 10-gallon freshwater tank with another molly, a platy and 5 newborn platy fry, all healthy. (: I'm planning on removing them once they get a little less teeny to avoid overcrowding.

Thanks for your time.


(Sorry for the bad picture quality... I figured it was better than nothing.)