[leebca]

The nitrate level you mention is not a concern for the specific marine fishes you've listed.

The condition is called a few different things. I like MHLLE best since it seems to be most descriptive.

Possible causes for lateral line erosion:
In order of what I think is most likely:
Vitamin deficiency (A and/or C); Poor Nutrition (including underfed and wrong foods)
Chronic Stress
Poor water quality (including high levels of dissolved organic matter and/or nitrites)
Activated carbon (either removing something the fish need or the dust clogging the pores on the fish)
Retrovirus
Hexamita
Amyloodinium-like dinoflagallate
Stray Voltage (is your system grounded properly?) [the article below disagrees with this one]

See this article for more info on MHLLE:
Marine Head & Lateral Line Erosion: A Description of the Syndrome and a Review of its Speculated Causes by Steven Pro - Reefkeeping.com

I'd like to see a picture of your fish to be sure we are in fact talking about MHLLE.

Saying you're feeding properly is not enough information for me. Tell me ALL the foods you're feeding. Are you adding vitamins to the food? If not, start adding marine fish vitamins to the foods. Feeding two times a day isn't also informative, I want to how how much food you're feeding. For instance, this is how to feed macro algae twice a day: How To Feed Macro Alage to Marine Fishes

As an aside, it is better to feed at least 3 times a day. I feed my fish 5 times a day.

See the one on the above list about stress? This could mean overcrowding the tank, or a tank too small for the fish you have, and many other things. I don't see overcrowding in that tank, but I would also like to know more. I want to also know more about the equipment you have, list all, how old the system is (when did it cycle or when did you set it up)? is there live rock in the system? what kind of substrate are you using? what type of mechanical filter are you using? any chemical filtration?

Water quality has many different aspects to it -- not just ammonia and nitrites and nitrates. I want to know the pH, temperature range, sp. gr. you're running it at. I want to know the test results for all the following: Alkalinity, magnesium, and calcium. You can do large water changes, but if your source water is tap water, that could be a problem. So what is our source water? How do you know your source water is pure? Then read this, too: What Is Water Quality?. Do you add trace elements to that system? A large tank requires additional trace element additions.

I think you can see a couple things from the above:
1. There is a lot to read and learn about this condition IF that is in fact what your fish have; and
2. There are many possibilities of what can be wrong INCLUDING any combination of those on the above list.

If you want to provide more information, we might be able to spot some problem areas, but it will take us knowing your system, the chemistries, and as much about your tank as you do now AND even more things about your system that you haven't been measuring or watching!

Good luck!