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.
These are peltiers. They get very very cold on one side and very hot on the other. You will freeze water on the glass where it makes contact. If you are going to use one of these, I would build and air exchanger outside the tank and use a rheostat to control the peltier's voltage.
I am currently working on cooling a tank with these and a modified swamp cooler design. They seem like they will work very well. Because they draw about 6 amps a piece adjusting the voltage with an ordanry rehostat may be out of the question. The other issue is providing that kind of current with a cheap easily avalible power supply. I just got a 35/30 amp MFJ power supply used for HAM radio (it is a bit of over kill but it was free). As for getting the Peltiers go on Ebay, I picked mine up for 3 bucks a piece.
Here is a link to how to build a small Peltier cooled terrarium. I will be using his logic curcit to regulate temp in my tank.
The more I think about the air exchanger, the more I like it. You could easily draw air from one side of the tank, pull it into a box with horizontal heat sink with fins connected to the cold side of a peltier. The hot side of the peltier would have a heat sink on the outside of the box. Then you could return the cooled air into the tank, thereby creating fantastic air flow and efficient cooling without the complications of water.