If the lights went out on your headlight tail light fish when he got sick will they ever come back on?

Yes, they will, although their is a five percent chance they will not and that their body has accepted the new change. When fish discolour or change colour it is a sign of stress, or being unhealthy. There is nothing to be worried about, for head and tail light tetra's it is very common for them to loose their brightness when sick, stressed or are not receiving enough light. You may have a heard the story that fish go white when kept in the dark, well for the head and tail light tetra this is true, but being put into it's natural environment it will go back to its natural colour. Although when sick it is best to have the aquarium light turned off. But not to have the fish in complete darkness, as they to need light to survive. There are certain chemicals and food that can be used to enhance the colour of your fish.