1. You just bought the fish
Sometimes introducing a fish into a new environment can cause color to fade, this can especially be a problem when fish are shipped to your home.
2. Water quality
This should be one of the first things you check if your fish is suddenly experiencing a loss of color.
The presence of ammonia or too high of nitrates can be toxic to fish, along with this other factors such as too low or too high of water temperature in the aquarium, to large a temperature fluctuation in the tank. Keeping your fish in too high or low of ph. Too high of nitrites or phosphates in the water can also cause color loss in your fish.
Testing your water as part of routine maintenance to make sure your water parameters are always in line with the species of fish you are keeping can help your fish to maintain its proper coloration
3. Poor diet
If your fish is eating well the problem could be diet. Feeding your fish incorrectly such as a diet too high or too low in protein depending on the species you are keeping can also be a contributing factor to color loss.Always research the species of fish you are keeping to make sure they are receiving the proper diet for their kind.
4. Tank bullies
Often fish that are being bullied or picked on by other fish will start to lose their color or may become so stressed they might start to darken in color or even appear to turn black.Keeping an eye on the behavior of the fish in your aquarium can help to determine if this is a problem and if fish need to be separated and placed in a separate tank.
5. An Underlying illness
Illnesses in fish can often cause a loss in color, if the loss of color in your fish is accompanied by other symptoms such as frayed fins, white spots, flashing (rubbing itself on ornaments or gravel) bloating, odd swimming behavior etc.The fish may need to be treated for an illness.Once the fish has recovered from the illness their color should begin to return.
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