Hi Lori,
I've seen these decorations, and while they are somewhat cute, I wouldn't put one in my tank! (my tank is only 5.5 gallons anyway).
A relative once gave me a bird feeder, because she knows I like to feed birds outside (not a pet bird). It was something from a discount catalog or something like that. It wasn't really something the birds would actually use. The perches were way too small, and the seed cake wasn't the right kind of seed -- the birds wouldn't eat it, and the squirrels and other undesirables would probably be attracted to it.
But I was happy that she wanted to help with the birds, so what I did was hang it outside. A few of the smaller finches sat on it and I took a picture and sent it to her. I then took the feeder down and threw the seed cake into the woods, away from the house, for the squirrels. I did feel a bit guilty, but I knew that the birds would not eat this seed and I didn't want more squirrels than usual, or worse, raccoons, to come and sniff it out.
So, just put the thing in the tank for a few minutes, take a picture, and then take it out. Send the pic to your cousin, and if he comes over, say that the fish kept uprooting it or something. Or say the cat got it! Cats are always good to blame!
LOL!
I did feel kind of guilty for not using my cousin's gift, but it wouldn't have worked out, and the same thing is true of your gift, so don't feel bad.