probably not in a tank, but you might be able to attach one to the outside of the glass. The way the Peltier effect works is to super heat one side of the media so it pulls the heat away from the other. These have had moderate success as CPU coolers in overclocked systems, but haven't caught on well because of cost and the fact that they tend to attract a lot of moisture. putting one of these inside a terrarium would heat it rather than cool it, but if you set it attached to the glass on the outside so the heating element was facing out, it might work. Keep in mind though these are designed to cool Computer Processors and electronics that get very VERY hot, towards 200F, so it might be a bit of overkill and a little bit difficult to moderate.