Recently, one of my Rosey Red Minnows has been acting ill. He developed the "death swim," and has been moving around the tank lopsided. For a while he had been lying on the bottom of the tank, hardly moving at all. I added some salt, and he is moving around a bit more actively now, but still not looking right. Before adding the salt, he devolped red spots on his sides. It almost looks like small spots of blood under the skin. I had not added any new fish to the tank, so I don't expect any contamination.

What could this be?
Parasites, disease, blood?
Could he just be dying?
What do you all think?

Fish health is not my forte. I've had rosy red minnows with blood splotches before myself. This usually only happens with newly bought batches that were getting poor care. It may be related to septicemia (bacteria in the blood that often gives fish blood streaks in the fins). As for the cause, it often is related to general care and water quality. If your water checks out fine, then it could be bacterial or parasitic in nature. There are really so many things that can be wrong to give the symptom of blood spots. Anyway, as I said, I'm no expert. I have saved many fish in that condition with salt, antibiotics, and good overall care with various water additives.

It could be as Robyn said, or it could be an internal infection. This is a common symptom of an infection although more commonly, it occurs with red streaks in the tail which was already mentioned.

Well he seems to be a bit better now. He still has the red spots, and is swimming a bit jercky. But he is upright and moving around. I guess I will just be keeping an eye on him for a while.

Might I add, keeping the water in pristine conditions will help more than anything IMO. Make sure the nitrates are under 20 ppm and ammonia and nitrites should be non-existent.