Filter sock very hard

[Mark 75g]

So not sure what is going on but just had a massive sps and montipora die off. All Soft corals, hammers, frogspawn and chalice are fine as well as the fish. Just went to change filter sock and it was hard as a rock. All levels seem to be fine what would cause the filter sock to get hard and stiff.
thanks
mark

[Johnic]

So not sure what is going on but just had a massive sps and montipora die off. All Soft corals, hammers, frogspawn and chalice are fine as well as the fish. Just went to change filter sock and it was hard as a rock. All levels seem to be fine what would cause the filter sock to get hard and stiff.
thanks
mark

Have you measured your calc?

[Mark 75g]

yes. It was reading 400.