A change in the pigment of a clown loach's stripe is an indicator that the fish is unhealthy or has been exposed to stress. Even though the stripe is a trait that all clown loach varieties share, the reason a clown loach can change coloration is not known.
Inadequate Social Interaction
Although normally their stripes are a dark black, you might find them gray or gray-blue. Stress is one of the primary causes for a clown loach's stripe to fading. One of the biggest causes of stress for clown loaches is when they are not given an appropriate amount of social interaction. Clown loaches are social fish who should not be kept in schools of fewer than five or six fish in a tank. Anything else will result in the fish being anxious and understimulated, resulting in poor health and fading stripes.
Not giving your school of clown loaches enough food could cause the stripes on individual fish to become more dull or change colors. If your fish are overall smaller than average, inadequate food is likely the culprit. Double-check the feeding recommendations for your loach variety, and make sure you are giving your fish the appropriate amount according to size and age.
Changes in Water
Changing the water in your aquarium can cause a color change in your school of clown loaches. Changing the water in an aquarium, while it's an often necessary occurrence, is often stressful to fish. If your fish suddenly change color after a change in their water, make sure you are doing only partial water changes, and double-check to make sure the chemicals in your tank are balanced as they should be. Give the fish a day or two to adjust to the new water.
When to Be Concerned
If your clown loaches' stripes suddenly fade, and no recent changes might have caused stress to your fish, make sure the pH and chemical levels are balanced, then call your local expert to have him help you discover the root of the problem. A sudden change in color without a corresponding change in the environment can indicate an illness or problem in your tank that needs immediate attention for the safety and well-being of your clown loaches and other fish.