For WC or F1 you may have to precondition your water prior to partials. Lake Tanganyika has a ph from 8.6 up to 9.5 and a GH of 11-17 dh and KH of 16-19 dh. Your tank is only 30 gallons so you may be able adjust your values in a separate receptacle (I used a 40 gallon plastic trashcan) for your partials without too much hassle. Many people will tell you that adjusting the water is unnecessary but for wild fish that come from such an extreme environment it may be the difference between keeping them alive and healthy or losing them in a frustrating failure. There are off-the-shelf additives for adjusting the GH, KH and PH that work well. Kent's makes a good product. Or, you can make your own.
Using coral sand in the tank will help keep the ph buffered up between water changes, but if your tapwater is too soft then your water chemistry will yoyo between partials and your fish will suffer.
We always desire to keep the chemistry of the tankwater as close to our tap as possible in the event we need to do a large-scale partial. In some cases, as in wild tangs, that may not be the best for the fish if the water isn't suitable for them.
I'd question the seller of the fish...what are the parameters of the water the fish are housed in? F1s may have been raised in more flexible conditions.
I'd also keep limestone rock in the tank, like SnakeIce suggested. It will help keep the high values stable as well. Texas Holey rock is one of my favorites for tang tanks.
Mark