Things in rocks that typically leach out are Ca+, Mg+, CO3, PO4, SO4, K+, Fe+++ and a few others.
Now if your gh and kH are shrinking I would test the pH of the water. If the pH is too high some of these things will precip out of the solution. Also phosphate can bind to certain things and make them precip out.
Honestly, though, I don't understand why such a large amount of TDS simply went away. Heh. Weird.
EDIT: Oh yea if you have a TDS meter. Use it and start the experiment again in a new bucket. Monitor the TDS everyday and see what happens.