Hey ya'll I just got a dwarf lionfish yesterday, I acclimated him for about 4 hours until the water in the bucket and the water in my tank were the same, I put him in and he is just either lying on the bottom or lying on the live rock, not swimming. I called the pet store were I bought him and they told me that this is normal, he is getting used to the new environment, but this morning he was still doing the same thing. My question is do lionfish swim often or do they lay around more than swim? 
....if your pH is at 6 that could have stressed him enough to cause his demise (especially if he was at a pH of above 8 in the fish store. Your other fish are still alive since they have acclimated to it (presumably they were in the tank all along and experienced it drop). I would suggest SLOWLY increasing the pH back up and then try again with another Lion. They are REALLY neat animals. I have two (pictured in my avatar, the Fu Manchu on left and Black Volitan on right). I see both of them daily (The volitan is out all the time) the Fu Manchu comes out when hungry and at night. Anyway so sorry to hear and good luck if you decide to get another one! Hope this helps!