I'm a newbie 14 months , so far no dead fish due to sickness, ( yes dead fish due to stupid mistakes .. even froze a whole tank of guppies...long story irrelevant, but not recommended )
But im absolutely certain I wouldnt still have any of my fish now if I didn't have an api freshwater master test kit, its been a lifesaver , the peace of mind makes the cost worth while. If ur ammonia is at a lethal level its invisible to the naked eye,
If ur nitrite is at a lethal level again its invisible to the naked eye,
Same with ph or nitrates ..
They only way for me to know is with testing, u can look at fish behavior but this can hit and miss ,
Certain commen behavior changes eg fin clamping and bottom sitting means lethal damage, if they get to that stage ur tank water is really toxic ,
With regular testing of ur water u will never see fin clamping , bottom sitting etc , u can fix the problem with water changes before it gets that bad .if u only have one tank , an api freshwater master test kit will last u a couple of year's, they are well worth that initial cost .