bad day at live rock

wow where do i start my local fish shop who i thought was a friend sold me some ph buffer and some nitrate and phos reducer had used the buffer and all seamed ok started to use the nitrate reducer and was told by the lfs to add 4ml to my tank every day for 6 weeks in a 150 litre tank so i started to dose the tank i got a fone to come home theres something wrong with the reef tank by the time i got home all my marine fish had died when i phoned the lfs to say what had happened he said to bring a sample down which i did he then tried to tell me it was a ph crash which i didnt believe as the colour in the tester didnt match the chart i would have said it was over 8.0 not under 7.5 which he said it was the water was very cloudy and got worse over the day till i couldnt see the back of the tank he did give me a credit note for the loosing all my fish but it didnt cover the total replacement cost that tells me he was in the wrong so either the ph buffer fail or it was a over dose of nitrate and phos reducer which i am thinking it was but i put in the amount he told me to as he know my tank size and set up