I don't grow in soil but I'd wonder about his water as well and I'd also wonder what the ph of his soil runoff is.

I wouldn't flush until I was sure the ph was abnormal. If it is....flush....if not....i'd switch to RO or distilled water and add Cal Mag as Wobster said. I'm just not sure that plant needs an entire flush......measure your soil ph first. Should be between 6.2-6.8.....and measure ph of water as well. It should be the same.