Those white spots look like ich to me. I'm no marine vet but I suggest looking into some medications as soon as possible because that nasty stuff can take over very quickly. Look up fish diseases as soon as you can and see if one of them matches your problem. Oh but be careful of what kind you get because if you have scaleless fish, they're very sensitive to a lot of medicines and can only take light stuff like Pimafix and Melafix. Inverts can't take any copper based medicine and are sensitive to a lot of others as well, so if you have any, separate them from the sick fish. I have a feeling it's ich but there are others that are similar and you have to see the behavior of your fish, not just their appearance. And with ich, medicine is important, but the most important is making sure the water is as clean as can be and do lots of water changes, but don't scrub the thing or throw out anything that could remove the beneficial bacteria. The redtail I'm not sure about but my guess would be maybe your ammonia levels are too high. Do you have a good filter? If you have a water testing kit, test for all the different stuff, not just ammonia (and if it's high, do water changes and add Ammolock afterward, it helps). I remember I had a tank a long time ago with guppies, back when I didn't know filters existed, and the ammonia levels got so high that they became lethargic and lost their color, then a lot of them died. Also, did you cycle your tank? Some fish are pretty sensitive to non established tanks and some don't do well if you don't acclimate them properly.
I should add I'm by no means a fish expert. Those were my guesses from my own nightmare situations, I highly recommend you research the different types of freshwater fish diseases and talk to someone in your local fish store who could tell you more about what the problem might be.
A forum community dedicated to Aquarium owners and enthusiasts. Come join the discussion about collections, displays, product reviews, accessories, classifieds, breeding, health, behavior, housing, care, and more!