Tiger Barbs usually get along with RTBS, but I think the 30 gallon is a bit small. Also, if you don't have enough Tiger Barbs (they like groups of 6), then they may be insecure and not their usual semi-aggressive selves. The shark may attack just be cause he senses that the Tiger Barbs are vulnerable.
Be careful when adding rocks or anything else from the great outdoors. Some rocks contain limestone which could kill fish. The rocks may also carry parasites, fungus, bacteria or contaminants like fertilizer that don't necessarily clean up with hot water. I make sure my rocks do not contain limestone and then I brush and boil, brush and boil, brush and boil.....you get the picture. LOL, it's not uncommon to find rocks or driftwood boiling in my kitchen.
Good luck with it.
Zambize