I've read books in the past that suggest using a heater under your gravel, or under your tank if keeping plants.
frozenoak said:I have them. There only real advantage is that they are hidden. The only thing you see is the cord that runs from the top of the tank to the gravel. That is easier to hide than the larger traditional heater.
I don't think there is a large advantage otherwise. It is theorized that the gentle heat coming from below the roots would help imitate the natural environment that aquatic plants thrive in, but I have doubts about this. At least in southern California where it rarely gets cold enough for the heater to be needed.
The big disadvantage, if you like to change your plants, would be that the plant roots entangle the cord. If you pull one plant it will most likely bring the cord with it. Now you have a cord above the gravel and it is a pain to get it buried again and any plants that were close by are probably half uprooted also.
In conclusion. If you are concerned about the ability to hide your hardware and you don't plan on changing anything around undergravel heaters might be your ticket. If I had it to do allover again I would go with the in-line heater or the canister filter with the built in heater by Eheim..
Hope this helped,