While it was still folded, we sprayed the filter with water. When it was dry, I made a template of half of a stingray and traced it onto the folded filter. I didn't get a picture of it on a colored filter, but here it is on a plain white one so you can see what I mean:
Even if you don't fold the filter before decorating it with dot markers, as one child did, it still looks cool.
Fun Fact: Stingrays only use their tail, which has a poisonous barb, in self-defense. Otherwise, they are very gentle animals.