First off, Welcome to AA, RJOVI
More information will be needed in order to properly help. What type of fish, size of the tank, how long has the tank been set up, water parameters (pH, ammonia levels...) and has anything new been done to the tank, or been added (i.e. new fish, new decorations...)? Also, how often is the water changed?
In the meantime, here are links to Freshwater Diagnostic Charts: