thanks KOTR!
I notice it doesn't work too well on a tall tank unless you have a lot of light. I couldn't get clear shots of the ones in my 30" tall tank. the ones here are in a shorter tank, about 22". I recommend doing this during water change. lower the water to 1/2, remove the tank lid, turn on the tank lights and you're set to go.
oh you can also have the room lights on to increase the lighting. Normally this is a big no no since you will get reflection off the glass when you leave the room lights on, but since i'm not shooting thru a glass with these dorsal views, it doesn't seem to be a problem. Also, my room light is almost directly above the tank so it works real well. In short, it's all about having enough lights.