Here's what you do. Put on a snorkel mask. Place you face into the water. While under water keep repeating, "Here fishy, fishy fishy." Works every time. Just don't let any one see you do this, they may call the guys in the white coats.
On a more serious note. I have had the same problem with some of my freshwater fish. I simply hold the opening so that it is facing the water and slightly out of the water, ever so slightly. The fish will usually fall out. You may also shake the rock to dislodge the nasty little critter. I usually just bump the rock with my free hand like pounding the bottom of a ketchup bottle.