Ok thanks U all are my true carnivore friends!!!, and sure all in the forums!,
The reason your under tank heater isn't working correctly is because your tank is not insulated. If you are using a traditional rectangular shaped glass aquarium you have 5 potential heating surfaces to work with; 1 bottom, 4 sides. At present you are only heating only 1 of those 5 sides and the other 4 are continuously subject to cool outside air making the cooler surface area 4x larger than the heated surface area...are you with me so far?
There are a few solutions to help improve the overall effectiveness of the heating pad WITHOUT having to put a pad on all 4 sides.
1) Get yourself some Reflectix silver insulated bubble wrap and wrap all 4 sides of the aquarium sealing the heating pad between the glass and bubble wrap. The heat won't be able to escape except through the top which should be covered with a pane of glass with a high power compact flourescent light on top. Most if not all of the heat will stay inside the tank. The downside of this is you won't be able to see the plants very well....but they'll be warmer.
2) Place your aquarium (without the insulation, with heat pad attached to one of the sides and a pane of glass covering the top) inside another much larger aquarium. You should also have your light inside here as well sitting on top of the glass pane on the smaller aquarium. Put a reptile hot rock or two in here as well..inside the larger tank, but outside the smaller one. You don't want any water in the larger tank (water and electircity hate each other!) Cover the larger aquarium's top with another larger pane of glass sealing in the light, hot rocks and glass covered grow tank with heat pad attached.
You should find an immediate rise in temerature that you can regulate (with practice) by sliding the top pane of glass over to let some heat escape...if you need to.
If you want to really heat things up then you can cover the outside larger tank with the silver bubble insulation. Again the downside of this is resticted view of the plants...but a VERY warm terrarium. This procedure works great for petiolaris type Drosera.
Just a few ideas for you to toy around with. Maybe someone can elaborate on these principals more creatively.