Don't let the light shine directly down onto the water surface, Bounce it off a white ceiling first, and keep the pan out of the direct glare. See if that makes a difference. (?) A pic of the situation with 2 flats, and compact flourescent:(I know not exactly the same, but you can see the bands are starkly visible this way. Light shining directly on the flats is very poor. perhaps this will enhance the water test to be viewed better)