If it were me I wouldn't even bother saying anything. Chances are, if it is that bad, they have noticed the problem and 20 other people have already brought it to their attention. I pick up shifts at my lfs and, although I know it's done with good intentions, after being told so many times in one day about a problem your already working on, it can get pretty annoying. The worst part about it is that the people who like to point problems out aren't the responsible fish keepers but the people who are there to pick up 50 feeder comets for the 2 full grown Oscars and the ID shark in their 55gal.