It seems like you don't have enough rock in the system. A tank that size should have about 60 kg to 120 kg of rock, depending upon the density of the rock. This can be made up of any kind of porous rock. If your not using 100% live rock, then you are going to need time for any dry rock to populate with bacteria. This can take quite awhile, sometimes a couple of months or more.
For a 3 month old system, you also have way too many fish in the tank. Typically you want about 1 inch of fish per 5 gal of actual water in the system. By the time you add all the rock and sand, that Red Sea 500 is holding about 100 gal or so of water, possible even less. This gives you a max total length of fish of about 20 inches. You have about 30 inches of fish. That's about 1 1/2 time the usual recommended stocking limit. There is also no room fro the fish to grow. You also have two tangs and the foxface, all of which are big dirty fish, and produce a lot of waste products.
I'd recommend you reduce the number of fish by about 1/3 to 1/2, so you end up with about 15 inches or so of fish. You can return them to your LFS, sell, or give them away.
You don't mention how you cycled the tank. It's quite possible your tank is still going through this process. Only time will really resolve this.
Now if it were possible to do this all over again, I'd recommend starting with only 1 or 2 fish, and after adding those, wait several weeks before adding another fish, and wait again. As matters stand, yoou are going to need to do fairly large partial water changes to keep the ammonia down. This will also slow down the cycle of the tank.