You can flush just fine with plain tap water if you correct the pH.
8 gallons is a very large pot for young plants like that. Generally, a 2-4 month old plant can live happily in a pot between a quart and a gallon. Anything larger, and you will have trouble with soil outside the root zone sitting wet since the plant can't get to it.
That's a CrAzY high pH- fits with the lokout symptoms but double check your meter calibration if you are using a meter.