I have a couple of hollow ornaments in my tank, one biggish. My concern is that the water will go stagnant inside them as no water can circulate. Will this harm the fish?
Good point.
But if the water can get in surely it can get out! so perhaps it will naturaly move with the tank currents because of the work your filters are doing.
I had an ornament that was completely hollow, never thought to 'empty' it every now and then. when I replaced it with some wood the smell almost knocked me out. So I think it's best to make sure to keep the hollows clean.