Ok, some more details:

1) He has been running CO2 in the tank driving the pH to 7.0 since the 5th of October - The tank is 2 months old and the fish have been in there for 7 weeks.
2) 125 Gallon Tank - was all of the Cichlids that died (about 10 of them) - tetras survived but were gasping at the surface
3) He adjusted the pH down to 6.9 on Tuesday evening. At the same time he added more fluorite, which clouded the water up, and added a bunch of fertilzers (not familiar with the products he is using). Thursday morning the cichlids were dead. He did not notice anything strange Wed...
4) Claims that his tap is testing 7.4 to 7.6 - but I live in the same area and the tap is 7.9

I think it may have been a mix of the fact that cichlids have a lower tolerance for this kind of thing, and the additional of flourite, ferts and some additional CO2 all at once... ??? I don't get how they didn't die on Wed morning though...