Aquarium temperature probe and thermometer
Tools for monitoring water temperature. There are immersion thermometers, digital ones with probe, and probes connected to controllers.
Detailed explanation
Temperature is the most critical parameter to continuously monitor. Even small variations (2-3 degrees) can stress fish and corals.
Immersion thermometers (coloured liquid in glass tube) are cheap but not very precise (errors of 1-2 degrees). Digital thermometers with probe are more precise and readable.
The probes connected to controllers (Apex, ProfiLux) allow: continuous monitoring, alarms for anomalous temperatures, automatic switching on of the chiller or heater.
The positioning of the probe: in an area with good water flow, away from the heater (which would give false readings), in the middle of the water column.
Having two independent thermometers is a good practice: if one gave false readings you can find out by comparing it with the other.