so bar,
would you sat thta the whitening is happening in the shaded areas first, or the areas protected from current first?
it looks like you keep a clean tank, you must scrape the coralline off, right?
if the equipment you are currently using is the same as the equipment from the last tank, it may simply be undersized for your current application. how big was that original tank?
two 150w bulbs may not meet the needs of that 500 liter tank, it looks deeper.
the same for the power heads.