Don't know about the TDS, but saf-t-sorb and other similar oil dri type products will bring down the Kh. This is only a temporary situation though, as the clay does have a saturation point. As a side affect of dropping the Kh, the Ph will also be lower for awhile.
Thanks for all the replies so far! What I was thinking is that I could have a container with oil dry that would be used to reduce the KH and TDS for acidic water fish, and which would be changed once a month to ensure continued efficiency. Would this be effective? (my water's TDS is about 173, but I have no clue what the KH or GH is)
Also, where can one find oil dri or similar products?