Not sure what your budget or soldering/assembly skills are but here's a temp controller kit
http://www.mtmscientific.com/tempkit.html
You can extend the sensor with a piece of 2-wire cable (or twist 2 wires together and heat shrink...or whatever). Coat the sensor in something waterproof like non-conductive sensor-safe silicone (auto parts store), heat shrink tube...whatever). You'll need a power supply (a 12V 'wall wart', aka power adapter) and a source of 110V AC to plug that into, of course.
The relay on that kit will handle 120V AC but only to 1A. If your pump draws more power than that...two options. Wire up an external relay with higher current rating, or find another 12V primary relay with higher capacity contacts (many out there for cheap) and replace the one in the kit with the larger one. The higher-capacity relay likely won't fit the same holes as the one in the kit but a little creative use of epoxy, wires, and solder can fix that.
If you need more precision than this cheap kit (and well you might since you're dealing with just a few degrees between OK and "NEED COOLING", Doc's right, you're looking at some bucks to purchase this level of capability. This would probably be a digital programmable thermostat. Hardware stores used to carry "analog" temperature sensors with copper heat pipe-based temperature sensors (like a cooler or air-conditioner thermostat). These often had contacts rated sufficient to handle your pump, but copper and aquarium life don't tend to mix too well.
I have a Peltier module-based cooler on my list of projects (well down the list, though). I have access to some killer heat sinks from swap meets and HAM days which will dissipate a "ton" of heat! The CPU coolers I just used on my LED lighting are remarkably small and inexpensive and dissipate around 100W which (depending on the size of your tank) could do quite a bit of cooling. You might check those out to mount a Peltier module to (or to even use as a radiative cooling device).
Anyway, good luck with that project.