that tang doesn't belong in your roughly 47-48 gallon tank. i'm sure you know that by now. they do best in 6' tanks.
light i wouldn't consider as part of filtration.
once a day is ok, unless you see a nutrient buildup, showing itself as an algae bloom perhaps.
herbivores should have some algae sheets to pick at on occasion also.
i would rinse the frozen food before adding it to the tank.
other that, you can make your own frozen food from fresh stuff from the supermarket if you wish. that's a lot cleaner than the LFS
meanwhile, i would be doing the weekly 25 liter change, and see what happens.