Basically, what you are looking for are detrivores-animals that eat detritus.
Of your eight tangs, are any of them the Bristletooth variety? Look for Ctenochaetus in their species name. I'm talking about Kole, Tomini and Two-spot tangs, among others. They have the right mouth shape to eat detritus off the sand (and rocks and glass). Most Pygmy Angelfish eat detritus too, so they're another option.
As for invertebrates, their are several good options. For starters, look for multiple species of snails that will reproduce in aquariums, like Cerith and Dove. They will multiply to reach an equilibrium with available food. Having more than one variety adds diversity to your snail crew, which is a good thing. Another good snail to have is the Nassarius snail. These guys eat leftover food, helping to keep the sand clean.
Brittle and Serpent Sea Stars eat detritus. Micro Brittle Stars will reproduce in your tank, which makes them my favorite, plus they don't get so big, like the non-micro varieties.
Pods also help. Again, getting multiple varieties improves diversity.
Sea Cucumbers are excellent detrivores, but don't make the mistake of getting too many, or they will gradually shrink and starve to death. Even in a 400 I wouldn't add more than two.
Fighting Conches are also great, but can also starve, so maybe 3 or 4. Maybe start with two and add more if needed.
I also have a few suggestions on what not to get. First off, the sand-sifting gobies. These fish remove all the life from your live sand, rendering it dead and unable to process detritus. Same with sand-sifting sea stars. I'd also recommend removing the six line wrasse. Wrasses in general are too good at hunting down the micro-life we try so hard to maintain in our tanks. You wouldn't put a barracuda in your tank. It would eat all your fish. Put a wrasse in your tank, and it will eat all your clean-up crew. I also don't recommend hermit crabs. First, they are inferior to snails, in the clean-up department, plus they kill snails to get their shells. Sure they can eat detritus and algae, if thats all there is to eat, but once they get a taste of fish food they are useless, in my opinion.
That's all I can think of right now. Hope this helps.
