Plain water unless your ph is too high rather than too low or your plain water is above 7-7.2. Ideally you want to keep ph adjusters to a minimum when flushing because they can build up in the soil.
Ideally all water should be left for 24 hours to allow the chlorine to evaporate.
I would put enough in to get a reading on each of your plants for ec and ph.
Flushing is putting 3-5 times the volume of the container in water through the pot to leach nutrients. It is stressful for the plant but sometimes necessary.
We will find out how bad your problem is when you post up your readings.
You basically flush then continue to water to 30 percent runoff until the readings look sexy !
I personally always water to 30 percent runoff to keep the media clean of salt buildup. House and garden nutrients also have a drip clean product which when used from the beginning of growth almost stops salt build up dead. I use it in hand watering soil and swear by it.
Yet again these are just my opinions so take em or leave em
Mouse