First don't panic and toss out the rock( or if you do...let me know and I will come by and dispose of it for you
)I have had 2 heaters break in my sump and act just like yours did. Did a large water change and ran some carbon. Didnt lose a thing. The third was curing rock in a bucket. I have no idea how long the heater was broke, just than less than a week. You might want to email the company that made your heater and ask them the same question you asked here.
Scott