Alex: If you really want to use heat tape...you should consider building a version of Jim's rack system at Capital Colubrids. Follow the link
I think heat tape for your current situation is not a good choice unless you intend to stack the aquariums or place them in a straight line on a flat surface. Heat tape may be cheaper but you have to take into account the hooking up materials. Heat tape is also not approved by the CSA. Not to say that i don't use it myself because i use lots of it including heat rope and they all do a great job. I think your best bet for just a few aquariums is heat pads.