Measure more than one place
Move the same thermometer briefly to the opposite end of the tank and compare readings. Avoid placing it directly against a heater or under sunlit glass.
Check at different times of day, especially when the room temperature changes. A single reading may miss an overnight or afternoon spike.
Improve circulation carefully
If a real difference persists, review heater placement and filter flow. Redirect water gently rather than increasing turbulence throughout the tank.
Do not add multiple heaters or fans without a plan. Confirm changes with repeat measurements and maintain the colony's conditions steadily.
For your own tank
Use the same thermometer for each comparison and allow it to stabilize at the new location. Check near the heater, opposite end, and surface only as needed. If the readings differ, adjust circulation gradually and measure again.
Use these as general hobby references. Test your aquarium and confirm product guidance before making changes.