Water parameters show no numbers...do you have a test kit? Ammonia should be 0, nitrite 0, ph around a 7.0 to 7.8ish, nitrate less than 40 ppm.
My best guess (As to the info provided) is your fish is suffering from a change in water quality OR you are noticing the veins in its tail.
20% changes ok, though my advice would be to up it to a 40 or 50% change.
If your fish is white, red streaks in the tail are more noticable than in more colorful fish. If its only a slight streak, it shouldn't be anything to worry about. In lighter colored fish you can see the veins better. However, if you see multiple inflamed-looking veins throughout the tail you should be worried. A spike in Ammonia or Nitrite can cause that, and its blood hemmorhaging throughout the tail.
I use my redcap oranda to moniter my water quality lol