The Fortress, without a chain, would hover about 5 feet below the
surface. Six feet of chain was enough to take it down, but I ended up using 8
or 10 feet with it.

I think Fortress states in their instruction sheet that you should use chain.
I think every manufacture states that you should use chain. I think every
anchor assembly article I ever read states to use chain.
S/V Express 30 "Ringmaster"
"No shirt, no skirt, full service"