what color is your water bowl?

try using a clear or white water dish, with only enough water in it so that the fish wil stay alive and breathing... make it so that they really have to flop and splash to move about.

Dekaybrown is right about the contrast thing... The better the snakes can see the fish, the better off you will be. If he is not noticing them, he either just can't see them, or he is too focused on something elsewhere to look into the water bowl (he may be more concerned with you than the bowl)... try surrounding 3 sides of his tank with cloth or paper so he feels a bit more secure, and maybe drape some foliage over top of his water bowl from the lid of the tank for some cover.