I have been feeding her frozen bits of fishes,shrimps,dried krills,flakes,boiled egg yolks.She wont eat any algae though.The water quality is a bit rough,nitrate at 20 ppm.Here are two of her pics the pic with full tail is of yesterday and the one with chewed to shreds tail is of today.
This pic you posted looks like a fish with no dorsal musculature left.
I think nutrition is your number one problem, possibly followed by bacterial infection, and water quality, as we don't know all of your water parameters.
I would stop feeding egg yolks and flakes today, and buy some frozen preparations for
omnivores, PE mysis, and frozen cyclops.
As mentioned above, buy a supplement like Selcon or Zoe/Zoecon. I would *not* consider this optional at this point.
Perform water changes, but make sure to match temp, salinity, and PH exactly. And mix new water for at least 24 hours. 50%, 25%, it doesn't really matter as long as you're improving the water quality. Which also means not using tap water.
If you want to reduce stress, do not change salinity as drastically as you mentioned doing. I would raise it back to around 1.023 slowly over the next week or so.
I would set up a small QT and if there is no improvement in a week or two you may need to treat for a bacterial infection. I'd look up fin rot... causes and treatments.