You tank may be overstocked, I don't necessarily agree, but doing water changes will remove that waste. You indicated you used to do 10% every week. IMO, that should be enough to compensate for a slightly overstocked tank but I am sure several on here will have a difference of opinion.
You are definitely over feeding. I never feed daily. At most I feed every other day. When I do feed the food is gone in less than 30 seconds. Tangs are grazers as are most other fish. Your LR will provide enough sustenance until your next feeding.
Phytoplankton - I used it religously for 5 years. For the last 2 I haven't. I have your basic reef setup (LS, LR, fish, coral, featherduster, etc) and have not noticed any appreciable difference in the health of my tank. Try curbing the amount you dose or stop altogether to see if that helps.
Lighting. I should have brought this up in my initial post. This can be a problem but since we know your nitrates are high I would rank this at the bottom of possibilities for now and until you rule out other causes.
One of the biggest mistakes hobbiest make is trying to eradicate a problem with chemicals and/or livestock. The less crap you put into your tank the better. Find the CAUSE of the excess nutrients and fix the cause and your problem will be solved.
I didn't see if this was asked, although it was definitely mentioned, are you running a skimmer? Skimming is important as it removes dissovled organics (ie waste) in your water.