Oil dri is a baked clay substrate material similar, if not exactly the same as Soilmaster. Both are made by the same company, I think. Soilmaster is known to drop the KH when first used, but I think it eventually stops doing that. Some have said this is due to its high CEC, but I'm not sure. In any case, what probably happened when you switched substrates is that the KH dropped rapidly, and that isn't good for the fish. You can add baking soda a little at a time to gradually bring the KH back to normal.
If I were doing this I would just try for a KH of about 2-3 dKH, no more. Fish will acclimate to that level very easily. But, rapid changes in KH are a problem.