Measurement
Thermometer
Stick-on liquid crystal strips read the glass, not the water, and are affected by room temperature. A glass or digital thermometer in the water is more reliable; a second one at the opposite end of a large tank reveals dead spots.
Short answer
An independent check on what the heater is actually doing. The single cheapest piece of insurance in the hobby.
What it does
- Measures water temperature independently of the heater
Strengths and where it fits
Strengths
- Very cheap
- Detects heater failure before the fish do
Good fit for
- Every tank
Limitations and where it is the wrong choice
Limitations
- Stick-on strips are approximate
- Digital probes drift and their batteries fail silently