Why do my fish keep dying?

So, we bought a fish on Tuesday, fresh water, lionhead, did headstands (which is a bit adnormal) and then died Wednesday night. We bought a new fish, name breed, but one the seemed happy and was swimming fine in the tank, and he died later that day. We don't know why they are dying. Our tank consists of pebbles, which we washed with warm water, a plant, which we purchased from the same pet shop, and water. This is all in a gl tank. We tested the water with a pH strip and the result was fine but the water still seemed a bit murky. What other reasons would there be for the fish to be dying?