First, and most importantly, what kind of thermometer do you have? If you have an external one (one that sticks to the outside of the aquarium), the temperature reading may not be accurate. Why? Because it is mounted on the outside, so variations in room temperature will cause the thermometer to have different readings. If you have an external thermometer, try taking a deep breath, open your mouth wide, and blow some warm air onto it. You'll find that the temperature will go up by as much as 6F!
If you have a thermometer that is in contact with the water, make sure it is working properly. There is always the chance of a thermometer that's faulty.
Another possible reason is that the heater is not strong enough for the aquarium, due to variations in temperature (outside). For example, even if I put my 500 watt heater into a tiny 10 gallon tank in the Arctic, the temperature of my tank would probably be below what I set it for! If you live in a cold place, you will need to get a more powerful heater.
Of course, there's always the chance of it not working. In that case, return it to the store for a refund/exchange.
Good Luck, and happy fish (turtle) keeping!
`Experienced Aquarium Hobbyist
· 1 decade ago