Angelfish come from warm water and stay healthy in it, commonly around 78 to 82 °F, held steady. That steadiness is equipment work, not luck: a heater cycles thousands of times a year and a thermometer is a small instrument that can drift or fail silently. Neither announces its failure clearly, because the fish show it first, through dullness, lost appetite or disease that finds a chilled fish quickly.
First: The Thermometer Read Against the Heater's Setting
Start by reading the thermometer against the heater's setting. The dial or display on the heater is a claim; the thermometer in the water is the evidence, and the two should agree within a small margin. A heater set to the chosen temperature that holds the water several degrees away from it is miscalibrated, aging, or undersized for the tank, and each of those has a different fix.
Second: Morning-to-Evening Drift
On the weekly check day, read the temperature twice: once in the morning and once in the evening, and compare the two. A modest settling overnight is one thing; clear morning-to-evening drift is a warning that the heater is cycling badly, the room cools sharply at night, or the tank is losing heat faster than the heater can replace it in the cold hours.
Angelfish feel those swings directly. Stable warmth inside the commonly used 78 to 82 °F range suits them; the same average temperature reached by swinging below it every night does not.
Third: Condensation Inside the Tube
Look closely at the heater itself, through the glass. Condensation inside the tube, visible as misting or droplets sealed within the glass, means the seal has failed and water is where electricity works. That heater is finished, whatever its dial says, and it should be switched off and replaced rather than watched for another week.
While you are looking, watch one full cycle if you have a minute: the indicator should come on as the heater warms and go off as it reaches temperature.
Fourth: Cable and Plug Damage
Run your eyes, and then your fingers with the power off, along the cable and plug. You are looking for cracks in the cable where it bends at the heater cap, nibbling damage, a plug with scorch marks or a loose fit in the socket, and any point where the cable sits pinched under a lid or hood.
Check the drip loop while you are there: the cable should dip below the socket before rising to it, so condensation and splashes travel down the loop and drip off instead of running into the plug.
Fifth: A Second Thermometer as the Cross-Check
Finish by cross-checking with a second thermometer. Two thermometers that agree give you confidence; two that disagree tell you one is lying, and without the second you would never know which reading your fish have been living at.
Replace a thermometer that reads apart from its partner after you have checked both in the same glass of water side by side.
Keeping the Weekly Round Shared
Five checks, ten minutes, once a week: the round only protects the fish if it actually happens every week, including holidays and the weeks life gets busy. The weekly duties sit in the Care Teams plan on KinsLe: Nadia runs the full five checks on Saturday morning and takes both the morning and evening readings that day, while Marcus covers the same Saturday round whenever Nadia is away, marking each check done as he goes. The written record of readings builds week on week in the same plan, so a slow drift in temperature shows up as a trend, not a surprise.
That is the real value of writing the round down: failing equipment is a pattern before it is an emergency. The Care Teams log on KinsLe keeps the readings, the drift notes and the replacements together, so the next pair of eyes starts from evidence rather than from a guess.