I'd have to say if you just had a rush of death in your tank, maybe learn from my mistake of last week and don't buy anything for a month, Then start with the hartiest fish you plan to buy, wait another 4 weeks or so and add another, and so on.
From the research and constant questioning I've been doing what the most common information I find is no tank no matter how great the numbers are is stable for the first year, and even less before the first 6months are up. Any change, from a water change to adding sand or rock or a fish can greatly make the system fluctuate and cause death, to include a fish going sick, from age, or disease.
That does give off anything from spikes in ammonia, or depending on the species even deadly toxins your tests wont see.
I also bought NovaAqua I believe its called same makers as Amquel. It helps bring up a layer of health resistance against disease and illness to your fish, replenishes their protective secretion layer quickly and gives them a mixture of healthy vitamines. It also removes metals and other toxics in water that amquel doesn't.
It's safe for all tanks and some even use it as a stress help when they add new fish during the drip process.
Now this is all stuff I've been told so take it with a grain of knowledge but if anything else I'd say time is a friend to all fish.