help?

[Blakskall]

i recently upgraded my 55 gallon to a 75 gallon with in the last month, and i had to add about 20 lbs of base rock to my tank (i know it wasnt the best idea), all of my fish were fine, and I'm not having any algae outbreaks to date so far, but i found my red striped angelfish bellyup this morning when it didnt show up around feeding time. I tested my water ammonia,nitrite,nitrate,phosphate all come up zero, (my 55 gallon was fully established) all of my other tank inverts and fish look fine. Do you think the tank upgrade had something to do with my angelfishes death? thanks