Yikes! I see some major issues with this tank. Firstly, you have fish that really are not suited for your size tank, so getting your nitrates under control and keeping them under control is going to be a constant challenge. Two yellow tangs in a 55G is asking for aggression issues. Pair that with the fact that there are multiple larger gobies AND damsels in this tank, and honestly, it's a recipe for disaster in more than one way.
Engineer gobies can get aggressive. Gobies don't mix well with each other as a general rule. I watched a yellow watchman goby kill his mate and then rip the eye out of another one of my gobies that was much larger than him, and this was in a 155. I wouldn't dare attempt multiple gobies in a smaller tank.
Your tank is 3 months old, stocked with the wrong fish, and too many fish for a tank this size. Your headed for a major disaster. Take the tangs back to the store, and get rid of one of the gobies. And honestly, if you can get the damsels out, do it now while they're still young. Damsels will absolutely cause issues for you and adding other fish as they mature, TRUST ME (or just google it) they're not called DAMsels for nothing.
Don't too large of a water change at once, as it can shock the tank. Smaller water changes over a few days would be more effective and safer. Nitrates don't really effect fish, although 80 is really high.
Start with better suited fish and better foods. Most flake and pellet food as well as brine shrimp (live or frozen unless freshly hatched) is garbage. Brine will entice a finicky fish to eat, but it has little nutritional value. Your goby should be getting the majority of its nutrients from sifting your sand, which is not established enough to support him in a 3 month old tank.
Sorry if this all sounds harsh, but unfortunately, it's the truth. I'm going to assume that you've gotten most of your advice from your local fish store. STOP LISTENING TO THEM, they just want to make a profit. There are very few good fish stores out there with knowledgeable staff.
