sorry hun, but be prepared for dead fish in the next couple of days. one will die after anther..

All of the previous answers are right. Check out Venice Girls hers is probaly going to help alot.
I sounds like maybe a fungus if not a parasite.
Check for more tell tell signs and if you want you can always feed them Pepso food and this will help ward off parasites even internal. Hope this helps too.

Parasites are small bodies of other animals that attach themselves to the fish and feed off of the fish. The parasites may be small, but they are visible as an attached object on the body of the fish. If you don’t see anything that appears like this, your fish don’t have parasites. Additonally, if fish have parasites, they will scrape on the rocks and gravel, etc., to try and scrape off the parasite. This is called, “flashing.” If your fish isn’t flashing, this is also a pretty good sign your fish doesn’t have parasites. This isn’t to say, however, that your fish doesn’t have an infection of some type. Your description is somewhat vague so, take a look at this list. It’s pretty good. These “small, brownish, see-through stains” could also just be a coloration change in their scales. Good luck!

a couple ways are drastic loss in color, unusual red marks, sugar like dots, suddenly lethargic, clamped fins (meaning that they’re tail and or fins are not fanned out but kept in) of course there are many other ways but these are some easy telltale signs

You could tell by looking and observing their swimming habits and eating. If you think it is less active and isn’t eating much, then I would be really worried.