Hi, I think I am one of the other people who has bought the same batch of Limpopo. However my set up is more akin to the TGM display tank which uses Limpopo and Amazonia. I do have tropica under the limpopo.
I was wondering whether you are injecting co2 or not?
After hearing about your pH swings I tested my system:
last night when the co2 had been running for about 4 hours = 6.8
Around midday today when co2 had been off for around 10 hours = 7.0
Tapwater pH = ~7.4
The colour of the substrate is what i found odd, but it was a gradual change over two weeks lightening up to what looks like light grey with black specs. I am hoping to cap the substrate with a layer of older batch limpopo in order to bring it back to somthing it should be; a dark substrate.