Nitrite is most similar to Ammonia as it is toxic to fish and it is effectively ammonia which has been converted into nitrite by a certain strain of filter bacteria named 'nitrosomonas' bacteria. Nitrate can be toxic at low concentrations to sensitive fish but most fish can tolerate 40-60 ppm of it.
I would go about testing your water for ammonia, nitrite, nitrate and possibly KH. You can test for these using an 'API freshwater master test kit'. An ammonia or nitrite reading above 0 is very dangerous to fish as it can irritate their skin/gills. Readings above 0 for these toxins should warrant removal of around 70% of your aquarium water to be replaced with fresh dechlorinated tap water.
It is always better to check for toxins in the water as these toxins may be causing the fish to lose it's colour. If there are no toxins in the water then it is more likely that it has contracted a disease which can only really be diagnosed and treated by studying a picture of the ill fish.