Thank you for posting the parameters. Sorry to hear about your yellow tang!
Some things.:
- You don't really need to bother measuring KH and GH, those kits are probably more for freshwater. When you make RODI water you remove the components that actually make up your KH and GH. These numbers are not really important in saltwater, as when you add salt to your tank you DRASTICALLY influence them because thats what is causing the general hardness. KH is tied to Alkalinity, and you'd be far better off getting a real alkalinity test.
- Your temperature is good where it is.
- I am assuming you are using the SeaChem Badge that goes in the tank for ammonia? These are good for quarantine with copper medications but aren't very precise. When you have the money I'd suggest investing in a more accurate ammonia test... Only trust the badge to indicate the presence of ammonia, not a real number. Any ammonia is not good, though not immediately fatal. It probably was a minor contributing factor to the Tangs deaths.
- Your nitrite being zero is good. Thats what you want.
- Your nitrate being zero is ~somewhat~ good. You don't want ZERO because your corals will starve, but I can hazard a guess its not a true zero. Its mostly being bound up or used in the algae/growths that you have in the tank.
- Firegobies are beautiful fish and are appropriately sized for your tank.
- Royal grammas are on the upper edge of what is appropriate for your tank, but this should not be a problem, so long as you don't overload the tank.
- Clownfish is more then fine in a tank that size.
The best advise I can give you right now is
stop adding things. Set yourself a timer for at least a full month, and just watch your tank... Observe the fish and their behaviors, watch as algae grows and fades, look for new growths, explore what you have but don't fall to temptation and cram more corals or fish in there. A young tank is a dirty tank that is prone to swings, and swings are what causes huge problems for invertebrates and corals. Fish can be more tolerant of swings, but they don't like it either!
Best of luck, and drop that LFS! You can still buy from them (I wouldn't support the business) but I'd research heavily before buying anything and don't take their word on whether or not something will be healthy for your tank. Remember they are a business... Some LFS' want you to succeed, some want you to buy stuff. The guy local to me out and out refuses to sell people certain fish if he knows their setups won't be able to handle it.