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Difference in temperature, radical change in TDS, etc may have done it.
Yes actually,.. but only a very tiny bit. I cleaned the glass with this tiny acrylic sponge all the way to the substrate and that did involve touching it.Just a long shot here...was the substrate stirred at all - even just a little bit? We're focusing a lot on the water, but if you had a gas bubble/pocket, or something in the substrate that was stirred up during the course of the water change, it might make it look like it was the water, when in fact it was something already present in the tank. I just can't see a 10% change of good water by the tests that you have done causing a reaction like that.
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