you need to find out what your basic parameters are. Usually that is the cause of the bleaching. Check your salinity, nitrate, pH, phosphate, ammonia. These are usually the cause of most problems in the tank.
Digis can usually handle tons of light and water flow. I agree with Phillip, check your water parameters because that is probably where your issue is coming from. A long time ago I had some stray voltage in tank (low voltage but anyway) and it caused several of my digis/montis to bleach. Once I resolved issue they all recovered within 2-3 months.
Good luck with your problem.
