It can be cured.
you will need another tank. acriflavine and formalin (5%), with salt.
the temp should be at 30degrees.
NO water changes for the first 2 days.
then do a 50% on the third day - top it up with water and salt. if the spots are still that bad - redose acriflavine (or melachite green) and formalin again.
If you have some snail rid, add about 3mL to 20L of water (copper is needed).
the above treatment is NOT your general remedy for ich, but after 2 weeks of hell and no results with any other method - this will work.
oxygenate the living daylights out of the water.