The majority of tropical marine life is accustomed to temperatures ranging between 25-29°C. A typical marine aquarium should run at a constant 26°C and would be expected to fluctuate slightly as the aquarium lights will radiate enough heat to raise the temperature slightly. This will cause no problem as the natural environment you are trying to replicate will also fluctuate slightly through day and night. An aquarium heater can be set to the desired temperature and works thermostatically to help sustain it. It is advisable to purchase a thermometer to check this as heaters often don't represent the same temperature as to what they have been set.