To be honest, you could be correct with regard to the flow being too much for cryptic, I haven't really researched that too well. All I know is the baskets and inside of the container are encrusted with those little tube worm structures and I find sponges and plenty of pods in there whenever I clean it & change the carbon bag. A while back when I was also running GFO in there, I used the top of an XP2 filter since the flow from that is less than the XP3 motor and I had read that GFO works better with less flow. That's the nice thing about the Rena XP series, the top assembly is the same size from the XP1 through XP4 and is fully interchangeable.
As far as sand in there, I haven't considered that for exactly the issue you state, I doubt it would stay in there and would likely destroy your impeller on the way through. I have read threads with folks running live rock rubble in their canisters, but no idea wrt crushed coral. You might be able to do that in a bag. I just found that anything in there other than the empty baskets and bag of carbon gets to be a maintenance issue if you don't clean it frequently, with the possiblility of elevated nitrates. Whenever I clean mine, which can be up to 3 weeds on occasion, there is a fair amount of mulm on the carbon bag and some crud in the bottom of the canister.
