A dying question?

[shimmy]

Im running currently a fluval 304 (I have a larger pump just not fitted yet) and I have a powerhead that runs 1600L/H no sump.
The tank is a 72 gallon bow front (48”L 24”H 12”W) I have about 60 pounds of like rock and about 40 pounds of fine sand (cant recall for sure how big the bags were).
Currently not running a skimmer im still trying to find one.
I finished moving all the rock around 14:00 and found them in trouble around 22:00.
My PH has never moved its always been 7.8 - 7.9 (that was tested as well after the wreckage).