I am an Altum keeper. I have kept Atabapos. When I brought them in the tank was pH 4.2 and the TDS were om the 20-30 ppm range. I have an RO?DI unit.
If you are serious about keeping altums I would suggest you join the Finarama site. Some of the best Altum people in the world can be found there.
There is a member there who worked with the senior Admin, and he has been spawning angels in the states for some time. I have som eof his fish, these do must better in tap than imports. He breeds the Atabapos. There are acouple of other members on the site who have managed to spawn Altums. Such people are very rare. I could not do it and would not even bother to try.
Also, that site often knows who has what for sale re Altums.