Once one of my Guppies was acting really sick, hanging at the top and clamping her fins and so on. I gave her 2 salt baths and then she was fine. So...try a salt bath. Use a container (I used my clean-water bucket) with tank water, or conditioned tap water at exactly the same temp as the tank, and put in several Tbsp of aquarium (not table!) salt per gallon. Sounds like a lot, but livebearers can handle salt, and Danios are hardy. Leave tham in there until they start showing signs of distress (starting to have a hard time staying upright)...although if they're like my Guppy they'll swim around happily until you fall asleep and plop head first into the bucket

I would guess the Danios will need to come out before the Guppies. I wouldn't leave them in there for more than maybe 15 minutes though.
Hey, I've been noticing that my Platies' poop is clear and stringy too, not red though. But they're not acting sick. What's causing this, anyone?