Take off the airstone. Just use the tube.
To be honest, I find it a pain to use those upside down bottles. I haven't found a foolproof method to make sure it stays upside down without my cats knocking it over.
I just use a glass bottle, dump in water out of my marine tank and some eggs, pop a bubbler with an airstone on it and put it on the windowsill. 36 hours later, BBS!
Descaps are ok, but it's great to see the fish demonstrating their natural hunting mechanisms and chasing them down.