What size is the dragon fish (by which I assume you mean Dragon Goby) and how long have you had it? They can reach up to 15"-24" but that number might be slightly lower in captivity and need a 55+ gal tank.
They are also brackish so I"m assuming you're adding salt and monitoring salt levels? What's your salinity now? What temp is the tank?
What does "safe" zone mean for ammonia and nitrite? They should both be 0. If they aren't, something is going on. Do you replace your filter media regularly (which isn't needed or recommended as it can mess with your cycle)? What are your nitrates? How much water do you change each week? Which dechlorinator do you use?
First thing is I'd do a 50% water change. Then get yourself a good liquid test kit (API master is best) and test ammonia, nitrite, nitrate and see what they are. Post here if you need help.
Sometimes fish die, they get old or sick, sometimes it just happens. But to know for sure whether something in your tank is harming your fish you need the proper tools.