Do I have to toss out my ink if it gets too thick?
In most cases, it depends on why the ink has thickened. If the ink has thickened due to excessive heat, then yes, it may be unusable because the ink has already partially fused or cured.
If the ink has just gotten thicker with time, then curable reducer can be used to help save the ink. Note of caution: Use it sparingly! Too much reducer harms the opacity of the ink.