A bigger problem is pure and simple BTU's --- look at how many BTU's most chillers or even AC units handle - that's how much cooling they can provide. A typical small AC unit is about 5000 BTU - a typical small fridge is about 150 BTU if I recall correctly.
Refrigerators and freezers are not designed for high BTU output, instead they are designed for insulation and gradual cooling. The also are not meant to be running non-stop.
I'll see if I can dig up some links, there have been a ton of articles on this, as well as much debate amongst the reefing community over the past 20 years or so. This is something a zookeeper and I researched back in the early 90's, when we were considering adding a tidepool tank with cold-water animals to the zoo.
Ah, here we go.
not the only link out there by any means, but pretty informative: http://www.beananimal.com/articles/d...m-chiller.aspx
Unless the tank is very small, the desired temp change is small, and the heat input is low (not alot of powerheads or other submerged pumps, and very cool lighting), a fridge style unit just won't keep up.
evaporative cooling is a better option - one gallon of water evaporated from the tank can provide something like 5000+ btu of cooling, if I recall correctly, and fans are cheap and easy to use.