Heater brand name: Tetra Whisper 2-15g Submersible
Says 2-15g but I use in 1.5g Tetra Water Wonders Aquarium and have used in 5g and 10g with no issues.
Adjustable (knob .. remote .. etc):
Preset to 78F
Submerged, price, suction cup, gallons range.
Non adjustable, prone to malfunctions.
If only option, use. Would recommend an adjustable heater.
Extra tips for usage:
Always test in empty tank for a few days with conditioned water and no fish. Monitor temps to ensure it won't malfunction. My first heated my tank to 98F before I noticed, but the 4 I've bought after that worked great. If I had another option for heaters I would use it but these work well enough for us.