What a good alarm does that a thermometer does not
A thermometer only helps the person who happens to look at it, and heater failures are cruellest at night: a thermostat sticks on, or a heater dies silently, and by breakfast the water is far outside the range your fish tolerate. A temperature alarm watches continuously and converts that silent failure into noise while the drift is still small and the fish are still fine. That early warning is the entire point — a two-degree excursion is an inconvenience you fix with a spare heater or a floating bottle, while the same failure discovered eight hours later is a tank of stressed or dying fish. For any heated tank holding fish you would hate to lose, an alarm is the cheapest insurance in the setup.
The alarm types, and which earn their place
The baseline worth fitting is an in-tank alarm unit: a probe in the water, a display on the glass, and a loud audible alarm when the temperature leaves the band you set. Audible alone is enough if you sleep within earshot and are home most nights. The stronger setup adds a second layer that pushes an alert to your phone, which covers the nights you are away and the days a failure starts while the house is empty; some heater controllers also cut power to the heater itself at an upper limit, which stops a stuck-on heater rather than merely reporting it. Two probes, or an alarm plus the ordinary thermometer you already trust, protect against the false comfort of one faulty sensor insisting everything is fine.
Setting thresholds from your tank's real range
Set the band from your tank's actual recorded behavior, not from the heater's dial. Watch the thermometer across a few ordinary days: most heated tanks wobble slightly between the heater's cycles and between day and night, and your alarm band should sit a couple of degrees beyond the extremes of that normal wobble on each side. Too tight and the alarm cries wolf on a warm afternoon until you start ignoring it — the most dangerous state an alarm can reach. Too wide and a failing heater is only reported once the fish are already suffering. Put the probe mid-water, away from the heater's immediate warmth and away from the filter outflow, so it reads the temperature the fish actually live in.
Testing, batteries and the 3 A.M. plan
Test the alarm monthly by warming or cooling the probe in a glass of water and confirming it sounds, and treat its batteries like smoke-alarm batteries: replaced on schedule, not on failure. Keep a spare heater, still in its box if you like, because an alarm at night with no replacement heater is only a notification of a problem you cannot fix. Decide in advance what the alarm means you do: heater dead means fit the spare; temperature climbing means cut the heater's power and cool gradually. Those thresholds and the tank's equipment details are exactly what a KinsLe Pet Passport keeps in one packet — the normal range, the heater fitted, and the records a helper or shop would need if the alarm ever sounds while you are away.