That reminds me of the undergravel heating pads that people used to swear by. I used to read aquatic digest mailing list back in the mid/late 90s and quite a few people used them. I never hear about it anymore though, so it's probably not necessary.
oh, only as an alternative to a submerged heater. And i'd place the heater pad outside & under the little fish pico or whatever.
I actually made a DIY undergravel heater when I was a kid... I didn't notice any improvement in plant growth.