How is it you have a design temperature of 0ºFarenheit, but you say you need heat tracing at -10ºC?  If your system is supposed to operate at -10º, don't you need a design temperature that goes to -10, or are you shutting down at 0F and only trying to keep the tank from freezing up at lower temperatures?  What do you mean?

I think FeCl freezing temperature can vary with concentration between -50 to -10 and the actual freezing temperature you will have is dependent on the actual concentration of the FeCl solution you have in YOUR tank.

If you don't want to freeze it, you'll need heating.

BigInch-born in the trenches.
http://virtualpipeline.spaces.msn.com

The literature values vary for the freezing point of 40 wt% FeCl₃ solution.  From the more reliable sources, I give the value as -14+2^oC.

To understand the variation in freezing point with composition, look at the solubility table & H₂O(ice)-FeCl₃^.6HO phase diagram given by Solvay:  http://www.solvaychlorinatedinorganics.com/docroot/chlo_inorg/static_files/attachments/pch_1610_0007_w_en_ww.pdf

H₂O(ice) and FeCl₃^.6HO form a eutectic, with eutectic point 33-34 wt% FeCl₃, -55^oC.  So, starting with pure ice, increasing FeCl₃ lowers the freezing point from 0 ^oC to a minimum of -55^oC at the eutectic, whereupon further increasing FeCl₃ raises the freezing point.  From data in Perry's, 7th Edn., the solubility limit is 42.7 wt% FeCl₃ at ^oC. Interpolation gives a freezing point of -15^oC at  40 wt% FeCl₃.  Pretty good agreement with the -13.5^oC given in the Solvay table.

There are 2 points worth considering:
1)  The slope of the FeCl₃^.6HO-rich liquidus at the 40% FeCl₃ composition is about 5.5^oC per wt.%.  
So rather than heating, you can lower the freezing point to -25^oC by increasing the water content by 2 wt%.

2) The liquid only partially freezes as long as the temperature doesn't drop all the way to -55^oC.  The fraction that solidifies can be calculated from the lever rule for phase diagrams.   Note that FeCl₃^.6HO is 60 wt% FeCl₃.
Example:  40 % FeCl₃ solution that begins freezing at -14^oC is cooled to -24^oC.  The liquidus composition at this temperature is 37.5 wt%.  The weight fraction of solid FeCl₃^.6HO will be (40-37.5)/(60-37.5) = 0.111 or 11%.
It is unlikely that this slush would damage your system, although it may be difficult to pump.  If the piping is buried in soil, it will be warmer, and thus less problematic.

Try contacting your supplier and get the freeze properties directly from him.

Different suppliers have have conflicting freeze properties becasuse they are making their products with different mixtures, perhaps even using byproducts instead of pure chemical. Some may have higher salt concentrations which would affect the freezing point.