MacGregorSailors.com

This has been calculated in another thread somewhere - in short, you cant add enough salt to depress the freezing point enough to be meaningful IIRC.

A rough Swag.....the "Molal Freezing point depression constant" of pure water is about 2 degrees C per unit Molality

This means you have to dissolve 20 grams of Sodium chloride for each kg of water (about 1 liter) or 80g for each gallon. Im dropping a factor of 1.5 here if you use rock salt but you get the idea)

for a 125 gallon ballast tank, it would take 80g * 125 = roughly 10 kg (22 pounds) to get your 2 degrees of 'protection"

thats a lot of salt.....

My raw water pump can remove water from my ballast there by floating my boat.
My first hand experience was ( and yours maybe different ) that using an air pump was arduous at best , I tried this when I drug anchor and ran aground . I tried using my small but powerful shop vac.... IMHO it didn't work very well. You have to really hold it tight against the vent for a long long long time. I was tempted to try a scuba tank hahahaha 3000 lbs of pressure and I had two on-board in the end I accessed my Sea Tow Insurance Policy