I'd be very cautious about adding salt in a situation like this. Dissolved salt decreases the amount of dissloved oxygen in the water similar to the way that elevated temperatures do. Both together might not be the best idea.
Mixer taps can be purchased as "pressure-balancing" version which are supposed to keep the outgoing temperature constant despite changes in the usage of cold or hot water elsewhere in the house from flushing toilets, etc. I have them in my showers and they do pretty much prevent unpleasant surprises there, but I still don't trust them completely for aquarium use.
I also have a pressure-balancing mixing valve in my basement, associated with the small on-demand tankless water heater I installed for my aquariums. Because my house has a pressure-tank water delivery system, which cycles from about 30psi to 50psi while pumping water, the pressure-balancing mixer valve still isn't perfect and I monitor it closely while doing fill-ups if using the water heater.
I hope you realize how lucky you were,
FINWIN
, that you caught this in time. I think you were minutes or even seconds away from disaster.