The idea behind these setups is that the heat from the PD transfers heat to the oil and then travels through the wall of the aquarium and then heats the water. One word of caution. for a small aquarium not much heat is needed to maintain a stable 78 degrees. On my 5 gallon I monitored the power consumption of my heater in the past and found it only needed about 7 watt hours a day. The laptop I am typing on right now is consuming 20watts per hour. That is a factor of 3 higher in one hour of what my aquarium uses in one day.
I have seen this type of setup a number of times on the web but I have never seen the temperature of the water.