some fluctuations in temp are normal in nature, as well as salinity. I think if you let your tank be too steady in temp/sal, when/if something does happen they are just way to sensitive. My tank temps and salinity varies quite a bit and everyone is always happy as can be (SPS, LPS, Zoa's and Anenome). Of course, I am not talking about when a heater gets stuck on and it jumps 10 degree's. Oxygen diffusion is another matter all together when it comes to temps.
Perfect stabilty is not always where it is at, you do want some movements but not wild fluctuations.
Some fantastic data can be viewed here
http://www.nodc.noaa.gov/GTSPP/gtspp-home.html
and
http://www.nodc.noaa.gov/dsdt/oisst/index.html
BTW, you can get that RANCO controller much cheaper. You do have to wire it on your own, but for most people it should not be an issue to do. Here is a good link to a supplier who will ship to Canada.
http://www.allthermal.com/Merchant2/...&Category_Code