How do you prevent water shock

Hi, how do you prevent "water shock" introducing new fish to a tank? I
followed standard proceedure of adding new fish. Is there something
I've missed? I can't think of what else could have killed my fish.
Details: I have an established 30 gallon aquarium (running 5 years with
3 small cichlids and pleco - so it's not a cycling or new tank problem)
and I added 4 new small african cichlids and 3 rainbow sharks. Iv'e
been checking the water frequently to make sure it was correct before
adding the fish and after (ph 7, gh 20, kh 9, ammonia 0, nitrite 0,
temp. 75). The new fish started dying the day after adding them, and
continued one by one over the next few weeks. They looked perfectly
healthy and were swimming around, grazing then the next morning, dead.
My last shark died yesterday, he looked happy and healthy the night
before, then, dead. The original fish are all unaffected (and I
strongly doubt they have been attacking the new fish, they ignored them
completely). I examined all the bodies and found no sign of disease,
parasites, paleness, physical damage etc, and neither did the local
fish store, they were as puzzled as I. They tested a sample of my tank
water and said it was perfect, and they can't figure out what killed
the fish. Can someone help with what it could be? Was I sold bad stock,
could it be undetectible toxins, could it be water shock? Someone
eventually suggested it could be this, how can I prevent it, and what
is it exactly please?