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.