Lfs is usually wrong unless you really trust them and it isnt a chain store, Ph doesnt really matter unless your keeping a Wild caught fish or african cichlids, which doesnt seem to apply to you. The deaths were most likely caused by ammonia and nitrite, nitrate is fine as long as it stays low. (0-10)
My Ph is at 7.6 aswell and my fish have never had a problem with it. (well my FW but my SW tanks water is buffered to 8.3 by my RODI unit) you can try and use indian almond leaves or beech leaves but as i said it doesnt really matter unless your fish are wild caught.
Look up nitrogen cycle if you are intrested about cycling. Good luck with the fish!
Thanks for all your help.