|The change in coloration you describe isn't a sign of disease; rather it's a sign of inadequate water conditions and diet. To bring your Red and Neon Blue Discus back to their vibrant coloration, you'll need to improve one or more of the following tank parameters: pH level, water toxicity, or nutritional requirements for Discus. Many species of fish may either lose coloration or darken when they are subjected to substandard water conditions.
Discus prefer slightly acidic water with a pH between 6.2 - 6.6, and an alkalinity (carbonate hardness) level in the range of 1 - 3 dKH. To achieve these parameters, use water you've conditioned using reverse osmosis (RO) and supplemented with an RO conditioner. Trace elements should also be added to the purified water to replenish the minerals lost during the treatment process. A liquid supplement such as Discus Essential or Discus Trace is ideal.
Other water parameters that need to be tested include the ammonia, nitrite, and nitrate levels. Discus can not tolerate ANY levels of ammonia or nitrites, so these values should be zero. Nitrates at concentrations above 20 PPM will cause Discus to lose their bright coloration. To lower nitrate levels in your aquarium, perform partial water changes and use a nitrate-reducing media.
Finally, the diet you provide for your Discus should include a balanced blend of meaty foods and vegetable matter. There are many dried, flake, and frozen foods specially formulated to meet the nutritional needs of Discus. You can also supplement these foods with a liquid vitamin or a garlic product such as Garlic Xtreme.
After you follow these recommendations, your Discus should quickly regain their coloration.