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Hey terp, I'm going to follow along as I have been curious about this myself.
Main advice I would give is to not top water. Because the autopot feeds from the bottom, anything that is unwanted by the plant/soil is moved to the top. When you top water it flushes all that bad stuff into the root zone. To get around this, turn off the res the night before you want to add the tea. Add the tea into the bottom of the autopot to the normal fill line. Do this a second time once the first go has been absorbed. Turn back on res.
*note: my experience is with autopots feeding salt based nutes. I'm not sure this would apply the same to organic soil.
This is one of those situations where the observed is not described by the proper process. Water will indeed carry dissolved nutrients in soil up to the point where evaporation removes the water and leaves the salts behind. This forms a zone or line in the soil with greater salt density than in the soil above or below this zone. The ions here will actively attract more ions in this salt region. It can become strong enough to be visible and toxic. Then when you top water this salt is washed back into the pot creating an out of balance situation. It can do the same thing in organic soil but takes much longer to form a salt rich zone.Just thinking about this. What mechanism of action is involved? How does a plant "move" substances to the top of a soil column against gravity? I assume by "bad stuff" you means excess nutrients?
Top watering to good run-off once a week will mitigate this situation.