Ok that Pajama Tang grows upto 15" and requires high water flow and oxygenation so you need to get that out of your tank asap
It is a very agressive fish when adult and will kill everything else in your tank if left in there as it fights for space.
The Yellow Tang grows upto 8" and is not really suited to a tank that small either as Tangs are open water swimmers and need space but you may get away with it but not with that Pajama in there
Your Nitrates are way too high, they should be kept at 10ppm or below, they should never be allowed to get to 30ppm, you need to do a water change now and change it once a week rather than once every two.
As for corals, you could try ricordae, more colourful zoos, torch/frogspawn/hammer corals, green star polyps, they will all provide more colour but until you sort out the water quality issues (and I can guess what's causing them) there's no point as corals do not tolerate high nitrates. The corals you have at the moment will look so much better with cleaner water, once that happens look to add more.