A GFI would certainly protect you against a specific type of electrical hazard. The serious danger would present itself when you tried to wire enough flexwatt (fed from standard duplex wall outlets) to heat a regularly sized aquarium.
The amount of energy it takes to heat water is tremendous. It is ridiculous to imagine the amount of heat tape it would take to heat a standard size aquarium, especially when it is recommended that the user keep the Flex-Watt at 95 degrees or less to reduce the risk of fire.
There is a reason that most household water heaters have a heating element that is rated between 4,000 and 5,000 watts. Buy a titanium heater that is made for aquariums. Its safer, cheaper, and people will take you more seriously.