Sorry to read about this. Most conditions such as this arise as a consequence of exposure to ammonia, nitrite or high nitrates. Let us know what your results are for each of these before you do a water change, plus nitrate levels in your tap water, and we'll be able to guide you from there. Also, can you let us know how you cycled the filter, what type of danio it is (a picture would be helpful and we can look at the red patches), how many fish there are and what type, and what size the aquarium is; answers to each of these will make it a lot easier to diagnose and suggest what to do. Thanks.