Ah ha, things are becoming clearer now. How did you cycle the tank during that 1 week? And why did you think it was cycled after only a week? (test results?) Unless you were using established media from another tank or a really good beneficial bacteria additive (don't think they even make one anymore) the tank did not go through the nitrogen cycle, hence the sky high ammonia, "fin rot" and dead fish. It takes 4-6 weeks to cycle a tank. Have you read this thread?
Cycling a tank It will tell you everything you need to know about cycling.
If you pH is truly below 6 (and who knows how far below) then there is a school of thought that thinks nitrifying bacteria can't even survive in that low of a pH. I don't know personally though, my pH is a rock solid 8.2 so I am not 100% versed in low pH chemistry.
pH should not be your biggest concern right now, a fully cycled tank is MUCH more important. Where are your ammonia, nitrite and nitrate levels now? Have you been adding an ammonia source (pure ammonia, fish food, raw shrimp) since all the fish died?
pH is a minor player actually. Stability is more important. Do you have a KH (and GH) test kit? With such a low pH I would assume your KH (and GH) is going to be really low too. A low KH means you will have a low buffering capacity, allowing the pH to fluctuate more easily, bad for life IN the aquarium but actually good for amending the water
before it goes into the aquarium.
When you were testing with the strips did you get similar pH results?