Am sorry to hear bout the fish.
If test was taken after discovery of the dead fish, and before water change,then ammonia reading an nitrite reading could be result of the dead fish.
Usually in mature tank's,the addition of one or two fish would pose no problem, but if biological filter was interrupted for some reason such as replacing a filter completely, or substrate, or if perhap's, non treated tap water found it's way into the tank (forget to use dechlorinator),or,, fishes are maybe being fed too much,then ammonia and nitrites can rise to toxic level's for fishes.
Some fishes are affected more quickly than other's.
In any event,water changes with dechlorinator as often as neccessary to reduce these two toxins is the way forward in my view.
Might also check to see that only one person is feeding the fish. When I was much younger,I lost several prized fishes due to three sister's that I was unaware were also feeding my fish.
Have also forgot to add dechlorinator on one or two occasions, but luckily I noticed the odd behaivor before it killed the fishes.