Just a little fish emblem with the word "CLEAN" on it.

I'm going to use this to indicate which filter is clean below my aquariums. I have two canister filters under each of my 3 aquariums. Only one filter should be cleaned at a time so as not to deplete the aquarium of beneficial bacteria. So, I'll Velcro® this emblem to the filter that was last cleaned. Next time, I'll know to clean the other one!