Terrible advice Tuesdays (T.A.Tues): An internal return pump won’t add any heat to your saltwater tank.
The rest of the story: Internal return pumps are specifically designed to use the water they are submerged in to cool the pump. The heat created by the pump has to go somewhere!
Keep in mind different pumps will give off different degrees of heat. Pumps that are higher flow will use more watts of electricity and with the increased wattage comes increased heat produced by the pump. Also, just because the flow rates of two different pumps are the same, that doesn’t mean the pumps will consume the same amount of watts and produce the same amount of heat. Higher quality pumps will be more efficient at producing their flow rates and will produce less heat than high-wattage, less expensive pumps.
When I switched from a mag-drive 9.5 to a eheim 1262 on my old 90 gallon tank, my tank’s temperature dropped by two whole degrees. The increased price paid for the eheim was clearly worth it.
If you are going to be running an internal return pump, expect your tank’s temperature to rise. Exactly how much of a temperature rise depends on a multitude of factors including efficiency of the pump, other pumps in the system, temperature of the room, airflow around the tank, etc.