Ok, simple thoughts: You can get three kinds of heaters for your tanks but before you do you need to consider what your filtration, lighting and airflow will be.
High precision expensive heaters with separate controllers and temperature probes.
Single body adjustable heaters that often say they're fully submersible but shouldn't be.
Fully submersible sealed heaters with fixed temperature controls (includes always-on low wattage heaters).
Note that, generally, Aqueon ships the "right" heater for the tank in their tank kits.
If you have a lot of air flow over the tank you're going to get a higher than normal rate of evaporation. (you will also get more if there is more life in it)
If you have lighting and a submerged powered filter you're going to get heat from both of those as well. They're fixed input rates of heat.
External filters and air filters don't add a significant amount of heat to the water.
There are a couple ways you can handle issues with tank temperature control. You can invest in a high value heater with high precision temperature control, you can invest in a pair of heaters of lower wattage with average temperature control or you can invest in controlling the environment around the tank and a single heater.
So lets examine the high accuracy control expensive heater first. Pro is that it should be reliable and it will cycle as needed and uses an external thermistor (a resistor that changes resistance with temperature) probe that checks the actual water temp. These are most commonly seen in professional installations. They're very reliable and come in a very wide range of wattage but they do have two maintenance concerns. They must be calibrated and the thermistor must be replaced every three to five years. These heaters usually cost more than we're willing to spend on the whole tank. They're great though. The simplicity of the heating element allows them to use a great deal less casing, reducing the over-heating range after shut off. They can easily be turned off when doing tank cleaning but are generally made for larger tanks than one you clean every few days. They're also a bit mean for betta and other "sitting" fish in that they suddenly get very very warm without warning. Some get exceptionally hot for very short periods of time and this can easily damage fins.
Two heater solution: Two of the same manufacture OR different manufacture set up as low range and high range heaters.
In my ten gallon boy tank during the winter I use a five gallon and a ten gallon heater. I set the larger one to run year round at the correct temperature for the tank but then also include the smaller one when the weather is cold at a four degree lower temperature. This means that the larger will overshoot to 81 degrees and the smaller to 77 degrees. You can also do this with two small heaters if your tank is smaller. I'd actually use a small fixed 7.5watt and a five gallon heater in 5.5 gallon tanks if I was worried about long time periods without my presence. This way your heater has a heater to support it and while the 75.5 degree lower setting isn't ideal, it will fight valiantly to protect your flower if the main one stops working.
Environment solution: A fixed heater is regulated by the tank's ability to dissipate heat. So a tall thin tank with lots of surface will lose more heat than a small square one. Round is warmer than cubic. Glass is warmer than acrylic. Closed is warmer than open. Sheltered is warmer than breezy.
Additionally have you considered UNDER your tank? The reason acrylic tanks come with little stands is the plastic is stressed by heat differentials. You can use a cork matting or place mat under your tank and reduce the stress on it as well as reduce the heat loss into whatever surface it is on AND protect that surface from dribbles. If you don't need to see your fish from all directions you can put backing that has been taped or spray-adhesive tacked onto cardboard behind the tank or on the sides you don't want your fish looking. This will drastically reduce the heat lost by the tank. Further reductions can be done using plastic wrap over unnecessary holes in the lid and blocking air flow in the room from reaching the tank. Remember your flower breathes air, so while blowing up a clear trashbag and putting the whole thing inside WOULD increase the heat efficiency, it wouldn't be viable for happiness... BUT it is something to keep in mind if the furnace quits.
This is why it IS important to select the right sized heater for the volume of the tank, the engineers include the "over heat" capacity of the heater when considering tank size but expect the tank to always be in a 72 to 76 degree environment with low air flow.
If you have an adjustable heater that isn't doing the job then try adjusting it by a little LITTLE little bit in the direction you want it to go. It might simply have a "bad spot" in the temperature control. If it doesn't stand a chance of maintaining your tank's heat because of the regular environment you're better off adjusting the environment. If the "right size" heater simply isn't enough but your environment is stable, get a second one and set it at a little lower temp and install it somewhere else in the tank. This second heater is what the small fixed thermostat tetra brand heaters excel at. Since they're very tough you can hide them in the plants.
Most heaters have a "waterline" mark. Ignore it, figure out where the top of the heater's casing is and submerge it as far as it will go and still not have a risk of leak. I'm saying to put it as far down in the tank as you feel safe with. The lower the heater is, the better heating it does - especially in tanks without circulation. Keep in mind that a heater set to keep the bottom at 76 degrees in a tank without circulation will keep the top much warmer.
Upright is right: Most heaters have their thermostat component in the upper body, if this portion is sticking out of the water all the time the heater will be trying to heat the AIR in the tank by making the water hotter. If you lay a heater down on the bottom it will try to heat the water in the rocks up by heating the water above it hotter than its setting. If you place a heater upside down it will also do this. This is why circulation in a tank is important, it keeps the whole tank at the set temperature instead of letting the top be hot and the bottom be cold.