Tank Heater for fish tank?

Hello all. I purchased a Tetra Heater from Wal-Mart to heat up a plastic rubbermiad container I have some tadpoles in now. It is supposed to heat up to 78.6 degrees within 7 hours, and the first one didn't (read 66) so I returned it and got another. Still says 66. There is just a oxygen bubbler in there and maybe 15 gallons of water(the heater is rated for up to 30 gallons and 100 watts) but the water feels warmer than 66. Do you think the plastic is causing it to not read the correct temp??