Log your tests
Trends explain what single readings can't.
Almost always water. Almost never the brand of food.
A tank is a small ecosystem that needs bacteria before it can support fish. Those bacteria take weeks to establish, and nothing sold in a bottle reliably shortcuts it.
Testing turns invisible problems into numbers. Ammonia at a quarter of a part per million explains stressed fish long before anything looks wrong, and a log shows trends that a single reading can't.
Then patience does most of the work. Stocking slowly, changing one variable at a time and resisting the urge to dose chemicals is what separates stable tanks from the ones that crash every few months.
Trends explain what single readings can't.
Cycling, stocking and changes all reward patience.
Proper conditions, quarantine, and a vet when fish are ill.
Twenty minutes, free, on your test results and your stocking plan.