That "egg crate" stuff is used for lighting if anyone is having a hard time finding it, just check your local hardware store in the florescent department.
Did you give up on the waterfall idea? It can be done properly but it is a bit of a project.
i never give up on it but changed how it works, i made a waterfall out of slate all down oneside (can see it on the fake rock steps) but insted of pumping the same dirty water round all day i set my mister to spray at it so when the waters gone it goes into drainage to throw away, which is much safer
ive seen him drink off it once since hes been in there too so he likes it