It looks like Septecemia to me - all the symptoms match your Tetra.
“Septicemia will often present itself as redness under the scales any where on the body. It can be concentrated in one area or appear throughout. It may appear more visible around the face. The discoloration is often impossible to detect in dark or red colored fish. Open wounds should be pinkish and usually heal quickly. A wound that is grey, black or fuzzy or has discoloration around the wound site might be infected and antibiotic treatment should be started. Septicemia sometimes shows up as a secondary infection to Popeye, an infection that leads to the bulging of one or both eyes. On occasion, it may lead to
Dropsy. Other symptoms may also present themselves in the form of sluggishness, loss of appetite, clamped fins or color loss. Observe your fish daily so you can learn to recognize his normal
behavior and be able to tell if he is acting out of sorts.”
That is the only illness I know of that makes the whole body turn red. (Now, if the fish has a scratch or something, a little part of it may go red. But not the whole body)
