Hi - a lot of it depends how / where you place your heater and what kind of water circulation you have in the tank (which will depend on what kind of filter you have and how it is set up). Also, the size and shape of the tank has an effect and the wattage of the heater. Describe your setup in a bit more detail and I am sure we can all make more educated rather than generalised suggestions :-)
* what dimensions is the tank?
* does it have lids?
* what filter do you use and where is it located - if canister, where are the inlet and outlets located?
* what size heater do you use, where is it located compared to the filter, and how is it orientated?
* where is the tank situated in relation to a draughty door or hallway or window etc?
* what are you using to measure the temperature and where is it situated compared to the heater?
With a picture from the above info, it will be a lot easier to give specific advice.